I think the only way they could've gotten more than 10 would be to make the grip thicker, but that would have had a negative impact on the M&P45's excellent ergonomics.
If you look closely at the standard 10-rd M&P45 mag, you will see that it is not a true "double stack", nor is it a "single stack". It is somewhere in between. ("stack-and-a-half"?)
If S&W had simply gone with a normal double-stack, staggered configuration, then the M&P45's grip would be just as fat, just as boxy, just as non-ergonomic as all the other hi-cap polymer 45's: Glock 21, HK USP, SA XD, etc. (And I presume the Taurus.)
I don't know about you, but to me, the S&W M&P45's ergonomics is the best thing about the pistol!