One reason is because if the amp itself was turned up too loud, it distorted the reverb too. So if the signal was clipping, then went to the reverb tank, well, what's a clipped signal then sent to a reverb circuit which was probably also sent into overdrive...
Ampeg's founder HATED rock & roll & distortion, even kicking bands out of the factory who's sound he didn't like.
Put up a schematic! If it's anything like I'm thinking, it's kinda like one of those "speaker-driven" line-outs - if my Bassman is crunching away happily, so is it in everybody's monitors.