Well considering how the 2d Megaman games were run to the ground in the last 10 years, and lost a lot of it's fanbase, they kind of had to make this one 8-bit to get the older fans attention. Look at how Megaman Anniversary Collection on the Gamecube and PS2 COMBINED, sold close to one million copies. While Megaman X Collection COMBINED, didn't even hit 500,000. Plus the best selling Megaman Zero game was the first one and it only sold around 400,000 copies worldwide. And all the sequels sold less and less, including both ZX games which didn't do very good at all.
Pretty much goes to show, it's the Classic NES Megamans that the gaming public cares the most about. This is because the NES games were much simpler for the average person to play, and could be enjoyed by everyone. When it went to X and beyond, it kept becoming more and more complicated until you get to the Zero series, which is impossible for anyone that isn't a die hard Megaman fan to enjoy.
Plus if Capcom used 16 or 32 bit sprites, it'd just remind people of Megaman 7 and 8, which are best left forgetton.