I guess majority of those early investors was only familiar with P2P. moreover, I don't think if anyone will take the risk to buy bitcoin using such amount back then, because no one knows about the good potential in future, so they wouldn't want to risk such amount in something that the outcome is not guaranteed.
It really depends on how much you could afford to lose back then, after all, Bitcoin was still a high-risk investment at the time. It didn’t have to be $1K; even just $100 worth of Bitcoin back then would be worth millions today.
Actually, buying wasn’t the hard part. The real challenge? Holding for many years without panic-selling. That’s where most people failed.