Soldier Field is a different situation altogether (Madden doesn't even have the rights to call it by its name), besides it was Northern Illinois who played their and they've only done so 3 times (with plans for a 4th in 2016). They play there against teams from the surrounding states that would exceed their small stadium (they get more money by playing a 'home' game in front of the other team's crowd at Soldier Field). Washington was only in Qwest Field (well CenturyLink Field now), for a season and 1 game.
If UMass is going to be there until 2016 then they should definitely put Gillette in the game (especially if it isn't very difficult to transfer a game from Madden).
It's also important to note that both of those schools had stadiums already in the game. Despite the fact that Husky Stadium was undergoing major renovations during 2012 in NCAA 13 you could still play there without having to navigate through construction equipment.
In the past EA has simply kept the stadium as it was until the new one was complete and they could go there and get all the info they need to render it in the game. In this situation EA won't be able to do that until 2014.
I read up on the details and the Stadium will be finished in time for the 2014 season, but the contract they signed with Gillette requires them to play at least 4 games there through 2016. Which means they would only be able to play between 2 and 3 games at their own place. So while EA will be able to have the new UMass Stadium in the game in time for NCAA 16 (they don't go to the stadiums until they are finished, so if it isn't done by a certain point early in 2014 they won't be able to get it in in time for NCAA 15), at that point UMass would still be playing the majority of their games in Gillette for 1 more season.
Also the likelihood of them updating UMass's stadium in the 360/PS3 version at that point is very slim (they will be focusing on next gen and making slight mods to the previous gen similar to what we saw with the PS2 version of the game).
Here's to hoping they put it in, and preferably that it's already in this year's game!