Back in Mass Effect 1 it made sense to only be able to play as a human becase humanity had only recently come into contact with aliens, you knew nothing about the setting, and you went in playing as a character who also knew nothing about the different alien races. So you and Shepard learn about alien culture, biology and history together.
But now we know all about the different species, have visited their homeworlds and have played as them in multiplayer. Depending on when and where the game is set humanity and it's relationship with the other species might not be as central to the plot as it was in the original trilogy. It might not be a major factor of the story at all.
The only real reason to have the restriction in the next game would be to make things easier for the developers, but easier doesn't mean better, and taking the leap and having multiple playable races like Origins and Inquisition could result in a better game. It would certainly help stop the next protagonist, whoever they are, just feeling like Shepard 2.0 and the story feeling like a rehash of ME1/2. We've seen the whole "humanity trying to fit in with the galactic scene" thing, it's time for them to do something new.
I also don't think it would be as difficult as people are making it out to be. Most of the extra cost would be on additional dialogue lines and voice filters. Many of the animations would already be there in the form of non-human party members and NPC's. There were mods for the Mass Effect games that could change Shepards appaence to that of members of the different species, and in most situations it looked fine.
It's not as easy as it sounds. No, dialog and animations are generally not the cost intensive aspects of games, but fitting all the variance of multiple races into every aspect of the game isn't a trivial task. Turians would need special facial animations for their mandibles and more rigid faces, salarians would probably need a lot of tweaking and stretching to get their animations right, quarians can't kiss on the lips, and krogan don't exactly fit everywhere. Effectively, BioWare would need to accommodate every additional race into every cinematic either by making a branching path or generalizing the scene. The former takes more effort, and the latter closes the door to some critical story paths. I can't imagine Cerberus would hire an alien, and I don't see a krogan sneaking around in vents.
Also, what's this alien's name going to be? Either we'll be stuck with a quarian named something like Shepard, or we'll have to deal with some incredibly generic epithet in even the most intimate of conversations. (I can only take so many "I love you, Captain"'s)
My point is that BioWare hopefully know what they're doing and will spend their resources intelligently. While multiple races would enhance the next Mass Effect, I don't see any need for it and given the drawbacks, I don't think it's the wisest decision.
I also think BioWare has yet to nail down a perfect protagonist. Other than differences in the romance palette and short background missions, the ME series basically ignores the characteristics of your Shepard. From an egalitarian perspective, that's totally fine, but I think it'd be interesting to delve deeper into gender issues, personal history, and player class. That wouldn't be as easy with more options to deal with.