Likely risk management. He was working with two other people, but the sudden change in that dynamic obviously didn't sit well with them. It's hard to realize it sometimes, but doing something on your own even if you do have customers can be pretty difficult and the probability of failure is pretty high.

What if something happens to you? Can you keep going? Is there anyone else available with intimate knowledge of the inner workings to keep things going with customers?

These aren't easy questions for a solo founder. It's certainly not impossible for one person to create and run a successful company, but the fact that you have no support and no one to bounce ideas off of obviously creates more risk. VCs don't just want to spend money, after all, they do expect you to succeed.

Edit: A few typos

Paul Graham explained his reasons in an essay: http://www.paulgraham.com/startupmistakes.html