One reason: alts were really beaten down. People took their BTC and moved it into cold storage and/or their own wallets to take advantage of a fork and the possibility of new coins.

Now, once we've reached the "ex-dividend date" - so those who owned at the time of the "fork" will have access to the new coins plus the old - their BTC is left gathering dust. Makes sense to put it back into alts.

Some folks who understand this better than I do will weigh in to tell me if I'm misunderstanding this. I lean on @jerrybanfield and @hilarski to gauge my accuracy here.