Therein is part of the issue: Silicon Valley is built on software, and software development is very different from medicine. Not trying to say one is harder or easier, but one is a much more unconstrained space of creation in which a really good, revolutionary idea and a strong mission can create a company, and in the other... a broad vision might simply be impossible. "We're going to build a social network online where people can post and see their friends' posts" is a nice vision that can be implemented in code; "We're going to diagnose lots of diseases based on a single pinprick worth of blood" is a nice vision that you can't actually do.
I think their is in incongruence to medical development cycles and software development cycles that will keep most tech money from investing too reliably.
On the other hand, the Boston and San Diego medchem startups are founded by established scientists in their 40s. Computers are quite the anomaly in the space.
The infrastructure requirements are a lot lower in tech. It's possible to be a college dropout and already have many years and projects worth of experience under your belt. It's pretty damn hard to do bio and chem without a proper lab.
The vast majority I have run across have more traditional educational backgrounds, many with advanced degrees through the PhD. It's just not as fun for journalists to profile the 37 year old PhD as the 19 year old dropout.