I think a lot of it has to do with the fact that they can retrieve dual lands. (disclaimer : I'm not an expert)

You could, for example, run a two-color deck with 4 dual lands and 12 fetchlands. You retrieve the dual lands first (if you don't care about Wastelands) and then you get basic lands.

Does that mean that fetch lands are 3 times as popular ? Yeah, kinda, but if you had a choice between using only dual lands or only fetch lands, maybe the decision would favor dual lands in many contexts.