Well, I always like to try to come up with sensible explanations for all the big plot-holes in Animorphs, and I've got one for this one, too. *pulls a strip from a roll of duct tape*
See, when the Ellimist fixed Elfangor's messed-up time-line . . . who says that he also gave Elfangor's morphing powers back?
Please notice, Elfangor never actually morphed again after that.
And it makes some sense, since the Ellimist had to prevent Elfangor from simply returning to earth and being with Loren again, which is what he very well might have done otherwise. So, I figure that maybe Elfangor didn't morph because he couldn't. There's always a catch, with the Ellimist, isn't there?