Hardware startups.

-They generally have a higher capital investment costs. Most high tech devices cannot be prototyped by a guy in his spare time, unlike a web app.

-Difficult to scale from prototype to production

-Most high tech manufacturing happens in Asia, which makes it difficult for US based startups to find and communicate with reliable partners

-High capital investment costs mean that it is difficult to decrease the per unit cost to the point where high margins can be reaped

These are just a few of the considerations, and aren't applicable in every situation.