Off the top of my head, I believe it was due to research done during the SARS epidemic in the early 2000s that trailed off as funding dried up when it was no longer an immediate threat.
Also, replying to the grandparent: vaccines per se are not necessarily safe out of the door, they're medical products and as such can have potentially many unforseen and troubling consequences. However, in the past all the unforseen consequences of any vaccine (whether still used, or since then retracted from the market due to problems) would become known a few months and a few tens of million people into the phase 3. That's when it is very, very safe to jump on board. And this is definitely already the case with all the popular covid vaccines.
To summarize: human data trumps everything in medicine, and we've had A LOT of data points.