Obama and Tucson is what's up.
Pretty much this. The M&P series a bit newer on the market than many other models of gun. As such there are less magazines out there already in the retail channel, and fewer aftermarket options available. That makes them rarer, and in a time when there's a bit of a run on hi-cap mags in general that translates to backorders.
Though I hate to contribute to the rush, I'd suggest that if you don't like a possible 10-round limit, buy your mags soon. The whole mess might blow over without any changes, but it might not. Either way though at worst you're looking at having a few extra mags on hand.
Personally, I've already bought an extra hi-cap for my P95, 2 more for my AR15, 2 for a Glock 19 (which I don't even own but figured it was a likely future purchase
), and when I ordered my CZ-75 I put in for 5 magazines with it. Can't hurt to have extras.
Either way though it's going to bite if they pass the legislation. There's a whole school of thought that can revolve around designing a gun of as high a capacity as is possible in a duty size pistol yet retaining respectable power. That entire area of development is likely to see less interest with the passing of such laws.
Particularly worrying are the versions of the possible bills which completely ban even transfer of existing mags. In those cases, weapons that are out of production that only came with > 10 round mags become essentially non-transferrable paperweight without aftermarket mag support (which may be non-feasible economically, or even if feasible, may yield mags of sub-standard quality vs oem).
Overall, just bad, bad, bad all around.