All sorts of reasons cause this. My theory is that, after the rehearsal mix was done and the instrument and vocals set, the band actually changed things during the live performance - i.e., turned up the instruments to the point where they obscured the vocals.

The other possibility, s I well know from many years in the business, is that a full marquee/stadium/performance area sounds quite different empty (rehearsal mode) than well full (performance mode), and that the sound engineers weren't compensating.

I had a similar experience with Southside Johnny and the Asbury Jukes at the Fairfield Halls in Croydon - absolutely ear-splitting - and spoiled what was great music.