They didn't just decide to "skip" long term effect studies. Long term effects can be studied only over a long term. No one wants to wait five or more years with the world in disarray.
Do longitudinal studies ever happen before a vaccine is released? Every longitudinal study I can find uses data from after a vaccine has been released to the general public. Stage 4 studies happen after the vaccine is in use by the general public.
“Even after the vaccine is approved and licensed, regulatory agencies stay involved, continuing to monitor production; inspecting manufacturing facilities; and testing vaccines for potency, safety and purity.
The FDA also monitors adverse events that may occur related to receiving the vaccine, including through its Vaccine Adverse Event Reporting System and Phase 4 clinical trials—optional studies pharmaceutical companies may be required to perform after a vaccine is licensed to continue to monitor safety and effectiveness.” [0]