There's plenty of money being made in flights but it requires serious volume. Hipmunk were never able to take enough traffic away from kayak, skyscanner etc. as their only real differentiation (agony search) wasn't a big enough pull (other than for a sub section of business flyers who didn't have a travel implant to outsource the leg work to).

They also didn't put any serious energy in to markets outside the US (much less brand awareness in Europe) and were late to monetise supplementary channels like hotels and car hire.

Regardless, it's true that flight search is a tough game. Big incumbents, tight margins, crazy web of legacy madness behind the scenes (tech and business side).