Unfortunately, I don't have hard data on it. But my guess is that there are two issues from a business perspective:

1) Productivity software is not something that is likely to spread rapidly (and cheaply) like a social network or a game. Unless you can pull off something new and clever, it's likely you'll need to pay for users. 2) It's tough to make it monetize well: it isn't something big companies tend to pay for, and it's still relatively unusual for individuals to pay for it.

That's a tough combination. May change, but I can see why investors are skeptical.