I imagine it's because GWR services a large section of the London/South commuter belt. Cliche though it may sound, there'd be an outcry of the first order if the service was as pants down there as it is up here. No one really gives a b**ger about us woad-painted northerners.
GWR services (on the London-y bit as much as anywhere else) are perpetually late and disrupted. To be fair, a great deal of this is not actually their fault - it's mostly down to the flakiness of the infrastructure despite the vast amounts of money spent on it in recent times.
Nevertheless, it's true that GWR are not the shambles of Avanti. Most trains run, often between the intended origin and destination and while scheduled timings are often more vague aspirations than achievements there's usually some correlation.
However, isn't this mostly just blind luck? As I understand it, GWR recruited lots of drivers (and guards?) pre-Covid - for services which either never materialised or have now been un-existed. The resulting nominal surplus appears to have fallen out as adequate staffing to run the current reduced timetable. So we don't yet see the "Shortage of Traincrew" shambles that prevails on many other operators.
The GWR mainline was a disaster area on Sundays for years pre-Covid with reams of cancellations of already grossly over-crowded trains due to no driver/guard as a matter of course. That also seems to have settled down, presumably for the same reasons. Or has there been some change in the contractual position of drivers and guards and Sundays? Either way, not as bad as it used to be, seemingly by fluke.