Eldar only have one dedicated transport option, the Wave Serpent.

All I can say is that it is too expensive and it is not an assault vehicle.  It does provide good protection for the troops it carries and it can have a good armament, but it just doesn’t do the job when it comes to assault troops in particular.  It could be said that the points cost would be fine as is, IF it had an assault ramp.  As it stands though, a Wave Serpent can easily cost you more than the unit embarked in it.  I don’t think that should be the case for transport vehicles at all.