The closest thing we have to a valid answer is that maybe Tenshinhan didn't perfect it until going back to train with Chaozu, but it would be more of a stretch to assume he perfected it in so little time. You could argue it was due to not getting a good angle to hit just #17 or #18, but he could've just told everyone to get out of the way after Vegeta's defeat. There's no real answer beyond it being a clear plot hole.
If referring to when he launched it against Cell, however, it was due to Cell being the main threat at this point, as well as Tenshinhan's pride as a martial artist not allowing him to just take the easy route when faced with a strong opponent.