Because BTC is and has been gaining momentum for the past couple of months and that slowly has been the "crashing" of alts. I hardly would call this crashing though, a crash would be much more severe of a nosedive.
That is true that when bitcoin price start increasing, people convert their investment from other crypto currencies into bitcoin, and that is the reason that when bitcoin price start increasing othe crypto currencies start decreasing their price, but still there are so many altcoins who have good potential and their price is also increasing from time to time, even if the bitcoin price is increasing too much.