It was not rushed.
We have 250 years of experience with vaccinations and when their side effects happen. And longer with viruses and their autoimmune effects.
After a day or three the vaccine is completely gone from your system, all that remains is the immune effects, the possible side effects are all autoimmune conditions. Those autoimmune conditions will either manifest in 90 days or they will not. There's nothing that hangs around in your body for 3 years and then randomly pops up. By running a trail for 6 months post-injection you will be guaranteed to capture all the side effects from a vaccine study. This is the same with virus triggered autoimmune effects which either pop up in 3 months or they don't (and I've actually had viral pericarditis that affected me about 4-6 weeks after I recovered from a bad cold).
Most of them will run longer, because they don't have a massive worldwide pandemic and they need to prove efficacy so they need enough people in the control group to get infected and that takes time. They were able to speed run the efficacy side of things because so many people were getting infected (we were worried at one point that the virus would largely disappear in Summer 2020 and negatively affect the vaccine trials).
https://www.uab.edu/reporter/resources/be-healthy/item/9544-...