Lots of rich countries over-ordered or ordered multiple vaccines incase a certain one fell out of favour.

Most people probably have had 3 doses and a couple of covid infections by now, so aren't as worried as at the start. It's also common for a booster to knock you out for a day or so after, so even if they're on offer you might just want to avoid that and accept the risk of a covid infection later on (which may happen anyway).

In the UK people under 75 (or at extreme risk for health reasons) simply aren't offered boosters anymore, so its just the way things are going.