A) Sexy - the idea of running a travel startup sounds exciting. It feels like an opportunity to periodically escape from the humdrum life of staring at a computer screen in the office all day, while still doing what you're best at.
B) A broken business - travel booking, planning etc is very confusing and flawed in many ways
C) A "big pie" to take a bite out of - it's a multi billion dollar market, so there must be opportunity in there somewhere.
The problem is, every tech entrepreneur has these same thoughts, and everyone has tried to execute on them at one point or another. It's a ridiculously competitive market, and for the most part the big $$$ for revenue are locked up by monolithic travel booking networks, that are only willing to relinquish a teeny tiny piece of the travel $$ pie to newcomers.
Then drop in this hard truth - recreational travel isn't actually all that commonplace amongst average people. The average joe goes on one or two major vacations in their entire life. So if you want to make money, you need to cater to either travelers with a lot of disposable income (the wealthy or retired), or business travel. Those niches are already very crowded but dominated by big, established, aggressive players, that you and every other dreamy-eyed tech entrepreneur eventually have to compete with for attention.
It's a long, nearly impossible battle to wage for anyone who hasn't spent their life in the industry. Winning in that space is a war of attrition competing with businesses with decades in the travel field, all in hopes of this grand and naive vision of striking it rich while on permanent vacation. Unwinnable. The white whale.