Funnily enough, I've just switched over to a new full time vendor after months with a very good, very reasonable vendor. In my case, it's not because the old vendor did anything wrong (and I may even still use them from time to time), but this new vendor, who I've already used twice, has a wider range of products and offers next day delivery. The previous vendor only offered first class mail only (which usually takes two days).
With other people, they may prefer different payment methods. They may have a package go missing and blame it on the vendor. Their old vendor may increase their pricing. There could be a number of reasons, everyone's case is different.