Network effects is as glib an answer as "got there first."

These things can be a factor, but are not an explanation.

I don't know why first mover advantage wouldn't be a good explanation for this case. Just because other companies squandered their first mover advantage doesn't mean they didn't have it initially. The big barrier to entry for a Craigslist competitor would seem to be brand identity since people want to buy and sell where they know other people are buying and selling. First mover advantage plays a big part in establishing that brand identity.

"Got there first" is an explanation. It's an explanation of why some American pioneers became rich in railroads and slightly later arrivals didn't, for example.

It's not the only factor, but it's an important one.

I'd say "got big first" is a slightly more accurate phrasing, but the gist is the same.