SWT retired the 442s because using the 444s and (effectively) isolating the 458s to the Reading line gave them a mostly uniform semifast/express mainline fleet of Desiros.
Porterbrook apparantly also offered SWT one hell of a deal to take the 458s back in time for the start of the February 2007 franchise (to meet requirements to add more seats in part) that meant a 458 was quite a lot cheaper for SWT to operate than a 442 (owned by Angel).
It's also been said before, but with the tph to Weymouth being part of the DfT spec for the South Western franchise, there were not enough 442s to allow all services to be operated with the fleet, and as
Via Bank says, a mainly uniform fleet of Desiros is a lot easier to roster in service, keeping a small subfleet of 458s on the Readings (as opposed to a small subfleet of 442s on Weymouths supplemented by some 444 diagrams).
If NSE had ordered another 4 442s (Even after Clapahm destroyed 4-REP 2002 I believe there were enough REP motors to do this) I wonder if even 28 442s would have been enough to operate an enhanced Weymouth service? If SWT had kept the 442s, I think they'd be like the FGW HST fleet interior wise, but without luggage stacks!