I would guess because the veggie burger market is basically nothing and McDonald's does not want the conversation to become "Is McDonald's going healthy?" before they are 100% invested into those new marketing avenues. The reality is the whole "fast food chains going healthy" hype has died down almost completely and those succeeding in fast food now are the gourmet fast food shops that focus on delivering quality food (but definitely not healthy).

IMO McDonalds have excelled when it comes to Coffee and marketing the McCafe brand and I don't doubt they could do the same thing with "healthy food choices" I just think the hype around healthy fast food has died for now and McDonalds is not willing to push it until the public demands it again.