My recollection (which might be faulty!) is that the problem goes right back to SWT's replacement of the slammers in around 2004-5. Having replaced the CIGs and BEPs on Portsmouth via the Direct services with 444s, which were well liked, SWT then decided to put the main line's 442s off lease. Instead of replacing them with 444s like-for-like, they instead ordered additional 450s (there were reasons for that at the time). Therefore most of the 444s were taken back off the Direct to run the main line, and Portsmouth services had unsuitable suburban stock dumped on them instead. It's been a source of much complaint ever since, not helped by SWR's "this year, next year, sometime, never" approach to dealing with it.