Because he's going against the #3 CB, safeties, or LBs and Tannehill has always liked using the slot WR going back to college. That's not to say Wallace wasn't a huge disappointment.
I'm doubtful production would change, reliability would almost certainly be better. You'd lose some big play ability, but going off of last year it might not be that big of a decrease.
Nothing we couldn't overcome, IMO. He's a niche receiver. And his niche doesn't really work here for whatever reason. Ask yourself this. Would Tannehill rather have another Brian Hartline or another Mike Wallace?
Seriously? For all his faults, Wallace got more open, more often down field than anybody I've ever seen. Had just half the passes been completed when he was not just open but wide open Wallace would've had 1200 yards and 10 tds. He's not perfect but I'll take 1200 and 10 any season and all he needed was decent down field throws to do that last season. You're seriously using 1 drive of a pre-season game to determine the passing game would be better without Wallace and hartline? Awesome!
How quickly we forget what our offense looked like when we had no speed at wr. Defenses bunched up inside 15 yards and gave us very little space for yac. Wouldn't help the running game either. Mathews, gibson and landry are all pretty interchangeable. Wallace and to a lesser extent hartline are not. Nobody else gets open down field as often as Wallace or with as much separation. He stinks at jump balls but when you have a 2 yard cushion you shouldn't have to stop dead in your tracks and win a jumpball. I look forward to Wallace having a monster season if tannehill improves his accuracy just marginally.