The actual answer to this is that, in addition to all the baggage he brings to her image from the sex scandals, President Clinton has clearly got a little "colorful" in recent years--and even the sycophants and morons in Hillaryland realize he's good in small doses for certain things but a fucking liability if given too much exposure.
If you look at what they've leaked to the political press in the lead-in to her official announcement, they've straight up admitted reining in Bill while making him feel included is one of the priorities of the campaign.