Tomothy,
There are several nice choices for medium game in the 1911. As you posted the 10mm. We also have the 45 Super and 460 Rowland. The Rowland runs right at the heals of standard weight, standard velocty 44 mags, so should be a no brainer for smaller big game.
As to your question about popularity compared to the AR15, I honestly feel it is sheer numbers. There are likley more rifle hunters out there than handgun hunters. A lot of more traditional bolt action hunters, like me, have probably found the AR in 556 to be a fun gun, with some field applications, some varminting and likley general protection. Toss in the expired ban, and lot of folks have acquired them just because they could, and might not be able to in the future. With all of these guns out there, the experimenters have figured out that it is quite easy to make proper big game hunting calibers that are an easy swap of parts. Presto, we now have a "reason" to own them.
I bought my first, quite by accident, for varminting. Then I found the boards, and became to understand that they really weren't evil
, they were fun to shoot, had applications for self-defense and were even more accessorizable than 1911's (born tinkerer). Then around the early part of this century, the makers really began to really figure out how to shoe horn some really decent rounds in the platform. There always had been some wildcatting. The ban ended in 2004, and things just flat snowballed. The 556 round always had a fair amount of controversy around it as a hunting round. Today the controversy still continues about the 556, but not the platform. I have a switch barrel affair in 22LR, 556, 6x45 (.243 light) and 458 Socom (45-70 light). To coin a phrase, I can hunt anything from mice to moose, ethically, with the platform. I can plink, and I could defend my home.
So, combine the sheer number of guns out there, with the amazing flexibility, and it is no surprise the AR platform has skyrocketed. I am a died in the wool 1911 lover, but I am guessing it will never have that kind of growth. Still won't keep me from toting my 460 Rowland, and possibly using it for the right application.
Craig