Content distributors are maximizing the profit from their products. Licensing them to streaming services before sales from Blu-ray/DVD level off would be lost money. A hit movie BR/DVD release will easily make $5-10 million/week selling half a million or more copies each week. Netflix can barely offer that to license a movie for its millions of subscribers for a year -- they're only charging $8/month per person after all and that has to pay for rights to every movie in the library.

Once they do get to the point where it makes sense to license the movie again, Apple might pay extra for an exclusive so they won't license it to Netflix too, or vice versa. You can expect streaming libraries to suck in certain ways, like time between movie release and streaming rights, for a long long time -- until consumption patterns change on a large scale -- because that's what maximizes profit.