Is security the reason that hydrogen vehicles haven’t taken off yet, or is it a question of infrastructure?

Well, security is fixable. It's hydrogen after all and not a heavy gas; in case of leaks it goes up not down. leaks are still dangerous as it explodes when being mixed with oxygen. the flames are invisible, you only feel being burned but you don't see it.

but the biggest problem is psychological: The Hindenburg, still

Just compare hydrogen cars with electric cars and you'll see why people prefer electric. CA has 33 hydrogen refueling stations, so many people in CA could get by with one.

Is it that much more dangerous than say LPG though?