I suppose because Thomson want you to check-in bags and to be fair, most of their target market is going to.
Ryanair for example will almost let you choose your own fare. If you are costing them more money then they will charge you more, a very fair structure IMO. They encourage people to not check-in bags, hence the larger cabin baggage allowence, because they don't want the delays and the extra costs involved (eg. delay and fuel costs). I think that those costs saving initiatives are less of a worry to Thomson.