Basically because the extra capacity they have is needed towards the London end of the Portsmouth route. A 12.450 carries significantly more passengers than a 10.444 - around 200 more if you include standees (even though there shouldn't be any on a longer distance service). That's what the whole argument has been about these last few years, and frankly it's getting boring now, and the local MP should know this.
There is an easy solution to the capacity problem though isn't there. Stop calling at Guildford...
Or how about moving Guildford so that it is on the Southampton/Weymouth route? Problem is we have the network that the Victorians thought we needed at the time...
I agree that standing on a longer distance train shouldn't happen as a matter of course, but the according to the timetable, the last stop is about 20 minutes from WAT on the semis, and less than 50 minutes on the fasts. At peak times, there are trains all over the country where passengers have to stand for longer than that and on stock much older than the 450s.
one solution would be to make the up Guildford fasts set down only which I would assume makes a GLD to LON ticket invalid. Maybe SWT hasn't got the guts to take on the Guildford lot.