How so? ATW and Chiltern have the same ownership and are far better in just about every way (ATW's "no growth" specification from the DfT notwithstanding). Seems to me that the quality of Arriva's franchises isn't at all consistent. I suspect that they're given quite a bit of autonomy and it's really just up to the specific franchise's management.
As something that's more relevant to this thread, I was actually on board the train in question (it's part of my daily commute) and was seated near Mrs Wafula-Strike as I was travelling with a bike, so I remember the journey quite clearly. I find it very surprising that this thread lambasts XC for their part in the problem, but EMT gets a pass.
The train in question has a 9-minute timetabled stopover at Leicester and was running more-or-less on time as I recall. That should be plenty of time to allow a disabled passenger to use the station toilets. Additionally, the train was a service to Cambridge, not direct to the stated final destination of Stansted (presumably Airport), a quick check confirms that changing trains at Leicester would have been no slower than changing later in the journey, meaning that even if the 9 minutes wasn't long enough, the 32 minutes between trains would certainly have been. The BBC article mentions that (and it correlates with my memory of the incident) "there was nobody to help her at that station" (and the previous article says "The next possible platform was Peterborough", leaving little doubt that it's referring to Leicester).
The fact that EMT (as managers of Leicester station) did not have staff available to assist with de-training a disabled passenger makes them just as culpable as XC in my opinion. The XC on-train staff definitely reported the problem to their control, so there's little doubt that EMT were informed (presumably the industry has mechanisms for alerting upcoming station staff to the presence of passengers needing assistance, even if the train and station are operated by different TOCs).