The answer, as usual, is money. Developing these tanks would be expensive. The reality of the current situation is that you don't need an ass-ton of ordnance on the wings to accomplish the mission, so losing two hard points to extend TOS is a worthwhile trade. You only need to drop one bomb to kill a target now, so bringing 2 plus a gun is usually more than you need anyway.
Future conflicts? Maybe so. The bottom line is (for a strike) you might need lots of weapons, but you probably won't need that much TOS. For XCAS,you'll either need more TOS and less bombs (lots of NTISR and not much dropping), or you'll be dropping so much that TOS isn't much of a factor anyway.
Bottom line is, it would be a "nice to have", but we don't get money for that kind of stuff in DoN.