These are the reasons given on a website called MyCruiseBlog:
"MSC has previously run UK departures from Dover, but this is great news because it opens up to a whole new audience. Getting to Dover from the North of England has always been a real issue, but Southampton means that your journey completely avoids the congestion of the M25 and the M20."
The line's UK and Ireland managing director Giulio Libutti said: "Ex-UK cruising is becoming ever more desirable and popular and we are delighted to be working with Southampton port in 2011. Southampton port is well suited to our new planned itineraries cruising to the north of France, Spain and Portugal. The infrastructure and amenities at the port and in the city make Southampton a desirable and attractive port for us, with great transport links offered by road, rail and air, which will better service our passengers. The move also allows us to better penetrate Southampton's local markets, as well as the whole of the South West, which is saturated with established cruisers and an increasing number of first time cruisers."
http://www.mycruiseblog.co.uk/blog/_archives/2010/1/27/4440232.html
No mention of the High Speed Train there. Possibly because there is not a lot of room for baggage. Travelled on it for the first time on monday and very impressed. An hour and seven minutes to St Pancras and very reasonable £31.80 off peak return. Airline seating but a couple of tables in each carriage, which my granddaughter and I spread our stuff out on. She had come down from Blackpool and I picked her up from Euston, which is just a ten minute walk up the road from St.Pancras (and Kings Cross is next door to St.Pancras of course).
It is certainly an embarrassment to think of cruise passengers walking about just about anywhere in Dover. No contest with Southampton which is a lovely place where I had the pleasure of living for a decade. Best thing would be to whisk them up to the Castle and keep the windows blacked out so they cannot see all the dereliction.