I've written three or four different explanations, but they were all TMI-the short answer is that the 12 bar is more simple, with fewer chord changes, so that there are fewer beginnings and endings to worry about, giving you a lot more freedom. Also, one of the cool things about 12-bar blues is that you can work out a short lead phrase and repeat it over all the changes to good effect--not so much with 16-bar blues.

Also important to understand--early blues was not necessarily less sophisticated music that early jazz--in fact jazz developed by adding elements of blues.