I also wanted to ask this but realized I only "knew" they were illegal from the Simpsons[0], which is maybe out of date and might have been inaccurate at the time.
Probably judges got tired convicting geezers who committed murder to win the tontine.
it was mostly because of fraud. if you are a banker seems quite a nice premise to sell your rich friends a tontine with them and a bunch of Bering sea crab fishermen in it.