Because most pros started playing tennis when they were babies and not too many babies are strong enough to rip hard and consistent 1HBHs. Once they got used to using the 2HBH, it was too late for them to switch and have to re-learn the 1HBH, which is a totally different stroke. The pressure to win from an early age discourages them from risking switching and suffering many losses while they're learning the 1HBH.
Exactly the same reason few play S&V. Not taught anymore to the kids.