#4 There's too much money in it
This is certainly a factor. The Shield are one of the most popular factions in recent WWE history and all three members are former world champions.
If Dean Ambrose does leave WWE after WrestleMania, this is a chance for WWE to not only give fans what they want but also to capitalize on merch sales and the temporary increase in ratings any reunion may generate.
#3 To possibly set up a Fastlane match
Face it, WWE Fastlane is one of the most inconsequential PPV's on WWE's calendar. A lot of fans usually don't care too much about it because we know not much will change in the status quo this close to WrestleMania.
If Roman Reigns is indeed fit and ready to return, WWE could book a match for The Shield against Drew McIntyre, Elias and Lashley at Fastlane. Reigns in-ring return and that too with The Shield, that's bound to generate interest.