They still played the songs in their original keys which was a lot tougher on his voice since he always went to the limit of his vocal range in the studio (from the NJ tour onwards the songs were always downtuned at least half a step).
Furthermore, Slippery When Wet rocketing to the top of the charts meant an enormous rise in demand for live shows. There were up to 5 shows per week and he simply didn't have the knowledge and training on how to preserve his vocals back then. He just gave everything he had on stage every night and that backfired quite fast.