This isn't meant to be exhaustive. There are so many reasons not to be Catholic, and I've written so much about it. But with that disclaimer out of the way, I'm going to summarize some of the primary reasons I'm not Roman Catholic:
1. Perhaps the primary rationale to be Catholic is the belief that God has singled out the Roman church for special guidance and protection from serious error, in contrast to the plight of Protestants, who lack that guidance and protection. With that in mind, consider the following: historically, popes, Roman bishops, church fathers, and church doctors, among other leaders in the Catholic church, believed in the historicity of Biblical narratives, believed in the traditional authorship of Scripture, believed that Biblical oracles (e.g. Isaiah, Daniel, messianic Psalms) were genuinely predictive. That held true up through the anti-modernist popes (e.g. Pius IX, Leo XIII) and the BPC under Leo XIII.
However, around the middle of the 20C, modernism began to gain the upper hand in church circles. I think it's safe to say that nowadays, most Catholic Bible scholars, priests, and upper clergy deny the traditional authorship of Scripture, regard biblical narratives as frequently erroneous or fictional, and consider Biblical oracles to be prophecy after the fact.
So the institutional and intellectual leadership of the contemporary Roman church doesn't think God protected Bible writers from error, and doesn't think God protected popes, bishops, church fathers, and church doctors for about the last 1900 years from mistaken belief in the historicity of biblical narratives, the traditional authorship of Scripture, and predictive prophecy. But in that event, what possible reason is there to believe that God singled out the church of Rome for special guidance and protection from error, if, by their own admission, he failed to do that for the authors of Scripture, and he failed to do that for popes, bishops, church fathers, and church doctors concerning their view of Scripture?
2. Rome redefined tradition. This began with Newman. It was formally adopted at Vatican II. Here's an illustration:
Before Mary’s bodily Assumption into heaven was defined, all theological faculties in the world were consulted for their opinion. Our teachers’ answer was emphatically negative... ’Tradition’ was identified with what could be proved on the basis of texts. Altaner, the patrologist from Würzburg...had proven in a scientifically persuasive manner that the doctrine of Mary’s bodily Assumption into heaven was unknown before the fifth century; this doctrine, therefore, he argued, could not belong to the ‘apostolic tradition.’ And this was his conclusion, which my teachers at Munich shared. This argument is compelling if you understand ‘tradition’ strictly as the handling down of fixed formulas and texts...But if you conceive of ‘tradition’ as a living process whereby the Holy Spirit introduces us to the fullness of truth and teaches us how to understand what previously we could still not grasp (cf. Jn 16:12-13), then subsequent ‘remembering’ (cf. Jn 16:4, for instance) can come to recognize what it had not caught sight of previously and yet w as handed down in the original Word," Milestones (Ignatius, 1998), 58-59.
When you see that you're losing by your own rules, you snatch victory from the jaws of defeat by changing the rules during the game. That's cheating. By redefining tradition, by contradicting how tradition was understood for the last 19 hundred years, Rome falsified itself by her own standards.
3. Rome has reversed herself on some major doctrinal issues. Take the possibility of salvation outside the church:
That's exactly what you'd expect from an organization that's not divinely guided. That doesn't enjoy divine protection from significant error.
That's exactly what you'd expect from a merely human organization that lacks foresight, making things up as it goes along, and having to abrogate established positions due to unforeseen circumstances, and replace established positions with new positions that were improvised on the spot.
Another example is how the Roman church is reversing itself on the death penalty.
4. Catholic apologists contend that human reason is too unreliable to interpret Scripture on its own. We require an infallible interpreter.
That, however, generates a dilemma for Catholicism. How do you determine that Rome is the infallible interpreter? Do you evaluate the documentary evidence for Rome? If the case for Rome depends in part on Biblical prooftexts and patristic protects, those must be interpreted. If, however, unaided human reason can't be trusted to arrive at the correct interpretation of the documentary evidence, then you can't establish the Roman Magisterium in the first place. You can only turn to the authority of Rome to interpret the Bible if you are able to ascertain that Rome has that authority. But at that stage of the argument, you can't rely on Rome's authority to establish Rome's authority, for Rome's authority must be a conclusion you derive from the evidence. As a preliminary step, you must be able to prove that by means independent of the Magisterium, for the (alleged) authority of the Magisterium can only come into play after that's been established. If, however, unaided human reason is too undependable to properly assess the documentary evidence, then there's no way to get from your starting-point to the authority of Rome.
In your fallible opinion, Rome is infallible. So opinion undergirds the Magisterium, rather than the Magisterium undergirding opinion. How can the superstructure be firmer than the foundation? The Catholic appeal is like a balloon of certainty sitting on a spike bed of uncertainty. Under slight pressure, it will pop.
5. To my knowledge, apostolic succession depends on valid ordination. And valid ordination depends on right intention by both the officiant and the ordinand. But intention is a private mental state. Only the officiant and the ordinate are privy to their intentions. So valid ordination is unverifiable. Any broken link in the chain will invalidate everything after the break.
6. In many respects, the Catholic church is already indistinguishable from the liberal mainline denominations. Modernism has infected the Roman church from top to bottom. There's some residual conservativism (conservative by traditional Catholic standards), but even that's eroding, like a sandbar at high tide.
7. The Biblical prooftexts for Catholicism are anachronistic or fanciful. Catholic apologists recontextualize their prooftexts by swapping out the original context and swapping in the context of Catholic dogma.