As cool as they are for us, for a lot of more casual collectors/fans (the majority) the Luke pilot outfits aren't in the "iconic must-have" looks of farmboy, Bespin and Jedi. Most fans simply can't even tell the difference between Snowspeeder and X-wing outfits.
On the other hand, the new/recent HT Luke Bespin seemed to sell out pretty fast (though yes its possible HT lowered numbers due to snowspeeder sales, though counterintuitively, they released two versions of Bespin.)
I'd also argue the Luke Snowspeeder fig was more of a middling grade, starting with something those casual collectors would have noticed first: HT simply reusing a pretty old, already overused, Luke sculpt instead of going with a noticeably better new Luke sculpt. That was not only previewed by snowspeeder release time that in theory could have been used.
That was the death-knell for Snowspeeder Luke for casual fans: "better Luke coming soon."
The fig also had skinny legs, weak glove sculpts (Maul re-use,) bad/inaccurate knee bandolier, too-skinny webbing straps, slightly oversized chest box, goofy-wrong grenade, ugly stand, and importantly, yet another kinda-accurate-but-not helmet (a little better than so-so SSC version) that missed the perfect opportunity to nail that iconic prop.
Had they done Luke Hoth instead (second-place to Bespin for ESB in that iconic outfits list, but much further ahead than snowspeeder) for 40th I think it would have done better and sold out, though not as fast as the Bespin did.