How did Craigslist become so popular (BONUS: Why the 1995 look)
SEO. Their Windows 95 type website has some significant advantages:
- It looks like paper classifieds.
- Simple sites are fast
Now having a crappy website and a decent SEO strategy is nothing new. Having users consistently generate content for you? That novel strategy is the only reason Facebook and Twitter exist on the map.
TL;DR: More pages; More long tail searches; Higher rank on Google. Win; Win; Win.