I certainly can't think of any reason in this particular case - some of the reasons quoted on that website are very dubious! It has been known on a few occasions for a Virgin service to/from the south coast to not call at Reading when there is a fault in the rear cab (e.g. Defective speedo) and by running it via Reading West Junction they can avoid the need to reverse direction at Reading.

Interesting that a train with a fault like a faulty speedo, as an example, was allowed to leave the depot like that. The rule book says it should not do so. If the fault occured during the journey, it should be taken out of service at the earliest opportunity. Seeing as the fault occured in the rear cab, surely the fault would have been known about before the train commenced its journey from the originating station? And therefore the TOC cannot care too much about providing a proper service.
I find it incredulous that a train was allowed to miss a major stop such as Reading - but not in the least bit surprised that it happened!
FGW seem to be increasingly allowing trains off the depot which are faulty and should therefore not leave the depot in the first place.