There's no inherent reason why being cash flow positive is such an important consideration for a venture capital firm.

A startup that's not cash flow positive right now but could be massively so in the future (or, atleast, the market expects it to be massively cash flow positive in the future) is significantly more valuable than a startup that is cash flow positive right now but with not a ton of room for growth.

In fact, placing the requirement of cash flow positivity right at the beginning of the startup would probably squash a lot of good (i.e. valuable in the long-er term) ideas.