It seems counter-intuitive, doesn't it?
I think the main thing is that they cost too many points for squads that aren't good enough at the jobs that units must now have in 5th edition. Fast attack units can't claim objectives and they sacrifice lethality for speed (ours do, anyway), and therefore, they get overshadowed by two high-mobility eldar units: the wave serpent and the fire prism. Basically, the wave serpent is an extremely good, very fast transport that is available to a very large number of our infantry squads. This makes practically any troops or elites choice as fast (if not faster than) anything in fast attack. That means that our specialized and effective infantry units like dire avengers, fire dragons, and howling banshees tend to overshadow their fast attack counterparts, which are based more on speed as a primary goal and killing/countering certain things as secondary. Additionally, my personal favorite heavy support unit (and that of many Eldar players), the fire prism, provides a reliable way to kill several unit types anywhere on the battlefield. The fire prism is fast, it has better range than any fast attack choice, and two of them can basically counter anything. I like to say, "I have a prism cannon setting for that," about any unit someone else is particularly proud of, and that underscores just how much of an overshadowing effect this unit can have. So it seems that our fast attack units are stuck between great specialized aspect warrior infantry and great generalized firepower.
Honestly, though, I think that having a good transport unit and a good fast tank should not automatically mean that fast attack units aren't worth it. Many of our fast attacks have well-defined roles and they have intrinsic speed that they don't have to rely on transports for. The main problem with them is that they just pay too high a cost for that intrinsic speed. it's the same reason no one would take guardian jetbikes if they weren't scoring units. Just think how cool swooping hawks or warp spiders would be if they could claim objectives, but they can't, so we don't want to pay over 20 points per model for them. That's just my long-winded take.