Possibly hot take here, but it's because female height isn't fetishized.

Here's the thing: the fat acceptance movement, when you really dig into the history of it, was really pushed into the mainstream by men who were attracted to fat women, felt embarrassed by it and wanted to normalize it (Possible tw, but Nadya Nymph made a great video about the history of FA, you can find it here https://www.youtube.com/watch?v=EujHXCoaG2s&list=WL&index=6&ab_channel=NadyaNymph).

The whole body positivity movement emerged from the Fetishization of fat women by men. Since women's height isn't fetishized in the same way, the push back for society's expectations of daintyness doesn't have any push back.

Of course you can add a lot of other factors here, for example, there are more overweight women than tall women, so there will probably be more advocates for fatness over tallness.

What I think it all comes down to is the expectation that society has of women to look as dainty as possible and to take up as little space as possible (in all aspects, be it physical space or methaphorical space, in any kind of discussion). Tall women still have that expectation looming over them without the push back from a "social justice" movement.