Frankly? I blame it on the filler arcs. The Thousand Year Blood War arc in the manga is still going on, and probably will for another few years. With the material currently present, chances are an anime adaptation would last about, say, 30 or so episodes, give or take. It would catch up to Kubo's work in no time, which would necessitate - you guessed it - another craptastic filler arc. Unlike other series, after the Bount arc Pierrot has made absolutely no effort to integrate the filler arcs into the main storyline. Any sense of flow is thrown out the window when the main arc is suddenly interrupted by what amounts to several months of what constitutes an insanely bad "what-if" scenario.
As far as volume sales go, Bleach is still outselling Toriko, which has been Weekly Shounen Jump's poster child for years now. And even that series has gotten canceled.
My somewhat baseless (if not logical) prediction is that they are waiting for the manga to end. That way they can wrap things up cleanly without an abundance of filler material. The closest example I can think of is Inuyasha: The Final Act, though hopefully any future Bleach anime wouldn't get the abridged treatment.
