I think it depends on the depot tags. Hengrove and, oddly, Bath seem to be quick to change them whilst Lawrence Hill are decidedly laissez faire about it.
In terms of Winterstoke Road, I would assume it was a financial benefit. It was a large frontage on a main road = prime redevelopment site and hence the appearance of a retail park.
Depots tend to get redeveloped when they are in prime locations like a city centre or a main road, when they cost a small fortune to repair and maintain (like some cavernous old tram depot) or when simply, there is enough capacity in other depots to amalgamate. One thing that I always thought would a sensible thing to do (and eventually Stagecoach Devon did it) was to build a depot adjacent to a park and ride. Aside from the odd car, the place is simply empty overnight for parking....just when a bus company needs it!
There was plenty of chatter about new depot locations on this board, and JF did float the idea of an additional depot in the Cribbs or Yate area. That died simply because there wasn't the money and as
@carlberry has said, the works at Lawrence Hill now make it unlikely that they'll move from there. Instead, they took the parking ground in the annex at LH (as I thought they might). Therefore, I think that HE and LH are here for the foreseeable and don't see anything else to join them