I think a large part is the moment you say a particular figure will always be in rotation a portion of the fanbase immediately moves to "oh, well I'll buy it when I have money then" and the reality then becomes you end up warehousing a lot of figures that move as though they're peg warmers even though they're mainline characters. There's also mold use for other figures which is a factor in at least some of them.
Like, there's practical and marketing reasons not to do it so I get it, but also I agree there should probably b a classic version of the top 4-5 characters every couple of years.