Two areas I've seen stymie school start-ups. As others have pointed out, starting capital is an issue in many cases. For those starting out with a program in a shared space, this is less of an issue - we get to build up our equipment and such over time, and the shared space doesn't usually require (or, in fact, allow) any upgrades. I started with an 8x8' mat space and my personal practice equipment, and used fee income to get everything else I currently have for the program.
Even when there's enough start-up capital, or with shared-space startups, I see instructors struggle with getting students. This is less an issue when kids classes are involved (those seem to be easier to fill), but a significant barrier to building adult programs. Sometimes this is mitigated by foot traffic in shared spaces (like a larger YMCA), but I've never found that helps much in the spaces I've used. For me, this is the biggest barrier - just getting enough prospective students to walk in the door.