The virus hasn't yet mutated enough to make the vaccine not work against it.
Besides, we already know there are a lot of ridiculous anti-vaxxers out there who will refuse the vaccine regardless. And a lot of those people are elderly or at risk!
The longer this situation goes on, the less palatable shutting down huge chunks of society seems to me to be. If this is going to be endemic, which a lot of signs and common sense reasoning points to, then the management of the disease needs to change.
You're asking why a virus would mutate more slowly than a virus that mutates extraordinarily fast.