My logical explanation for why girls flake is because they have no idea who you are. In most cases, I'm some random guy that approaches them on the street, campus, bus, bookstore, wherever it may be, and it's difficult for them to go out with someone their not familiar with so they just flake.

Now when I text them and I start to get the vibe like they're uneasy with the whole meeting a stranger thing I text something like this and it's been pretty successful, "If you're having mixed feeling and prefer someone you're familiar with it's cool. It's hard meeting a complete stranger." This displays that you're not needy, and at the same time you're telling her she's not bold or brave and just follows the traditional boring way of meeting new guys.