Because while Richard lived the majority of the traditional Yorkist faction followed him, omitting those like the woodvilles whom he had screwed over to obtain the throne. I don't think that Richard was ever unpoplar enough that the majority of the Yorkist faction would abandon him for a pretender with no proof of his authenticity, so the candidate would need to appeal to the Lancastrian faction to work most likely. Henry dd just this, and while his marriage to Elizabeth was a serious attempt to make himself into a candidate for both factions, it is telling that the most serious uprizings against him were in favor of Yorkist claimants, meaning that the nobility always considered Henry Lancastrian.
Of course, that may just reflect the fact that the WotR had left a distinct shortage of credible Lancastrians - which of course was what had given Henry Tudor his chance.
In the event, though, the most serious revolt against Henry VII (and against his son) was not dynastic at all. It started in Cornwall, which had been Lancastrian during the wars, and was directed against Henry's policies (on taxation) rather than his family. The rebels might have supported Warbeck, or he them, had he ever shown up, but in the event it ran its entire course without benefit of any Pretender.
The converse situation prevailed in Kent. Though generally Yorkist during the wars, it never gave Henry (father or son) the slightest bother. It had been the jumping off point for Richard of York's return in 1460, but when Warbeck appeared in 1495, showed so little interest that he never even dared to land, and those of his supporters imprudent enough to do so were seized by local militia even before the king's troops could arrive.
Ditto in the next reign. The only serious rising against Henry VIII again related to policy (this time religious) not to family. Even when it was at the height of its apparent success, no Yorkist claimant ever raised his head, nor did the Pilgrims show any interest in one.
In short, support for or opposition to the Tudors seems to have had little to do with former dynastic preference. If any pattern can be discerned, it would seem to be geographical, with Cornwall and the North disaffected, the south and east loyal. Wales, of course, was an exception that proved the rule, outlying but loyal, presumably because the Welsh saw Henry Tudor as one of their own.