That is the definition of luck. Luck isn't magic; it's being in the right place at the right time. Maybe a big site randomly stumbled upon your site one day and linked hundredss of thousands to it. Maybe you made a site about blue widgets and suddenly every celebrity suddenly loves blue widgets.
I've developed great sites that made next to nothing. I've developed crap sites that made tons of money, and most of those sites I lucked out on. I got in at the perfect time or got some links from sites that I probably didn't deserve.
There IS a lot of luck involved when it comes to hitting it big on the internet. If you go in to your idea thinking just because it's flawless you can't fail...well, you'd be wrong.