Given how bad a train can look after just a few hours in service, the Electrostar interiors must be particularly hard to keep clean compared to a rather clinical and sparse 700. A 700 can be cleaned easily*, whereas the seats, carpeting, and loads of nooks and crannies on a Electrostar must make it very hard and time consuming.
Deep cleans are one thing, but with cost cutting measures now the norm, by the time there's a deep clean the train has probably got very dirty.
* Toilets excepted. They're a disgrace.