While it is easy to modify, it still takes time to test the modified vaccine. The vaccines were designed to target the spike protein which is shaped to allow it

While it is easy to modify, it still takes time to test the modified vaccine. The vaccines were designed to target the spike protein which is shaped to allow it to enter human cells. If that protein evolved too much to evade the vaccine, it would also become less effective in entering human cells. In the future they might prepare a wide number of vaccines targeting common coronavirus ahead of time so there can be quick response when an outbreak appears.