The thing is, once he's done in the subspace, his memories of training there kinda start to fade ending up a bit like dreams.
Anyway, I was weird out by his coming home thing, but once we find out his mom knows the hermit, things got interesting.
I guess Drakken needed Rebecca's power to go back to his prime, but he didn't need Shiki's because he's strong already.
Or maybe there's a compatibility issue.