While I have to agree that it may be "convenient" to link to a BT device for control.. BT is sketchy.
One thing I could imagine happening is that I'm in the middle of a set, reach over to my trusty BT device to make a set setup change, and <insert very loud tire screeching sound here>!
Granted, wired solutions can (and do) have the same issue at times. But they are FAR more rare then BT solutions.
Hey.. I'm all for convenience and "quickness" of access to setups. But tech is tech (and it's my full-time job for the last 20 years). Things fail - and the last thing you want to do is put yourself in the position of being subject to a weak technology process.
Also, can see Line 6's perspective here. If they include it, pros (or semi-pros) will think they can rely on it. First time it fails during a set will cause a HUGE wave of bad publicity for the product... when, in fact, it's the base technology (BT) that is flakey by nature.
Anyway.. just my .02