Because if you 1) have enough RAM and 2) between two boots you use less files from the HDD than the size of the RAM then you'll observe the speed up from the SSD exactly only once between two boots. Every modern OS uses the RAM cache like that -- once you actually start to work, you work from RAM. On another side, those who buy SDD "for speed" are the same who when asked "but SSDs are significantly smaller" answer "I have everything in the cloud." Absurd, isn't it?

Edit: The title of the topic is "why are you not using a SSD yet," I give my exact arguments and get downvoted. Eh, crowd.