You have to wonder why FR has not added CWL routes it must be yields are not as good as they should be.
Surely just because one or two routes do well doesn't mean another new route would?
TFS and FAO are big demand routes and very underserved from CWL. As Harry has said FR probably know there's sufficient Welsh passengers using these BRS services to support CWL flights. TFS and FAO are good routes to build ancillary revenue. Those type of passenger will buy a bag, will pay for golf clubs, will buy a drink or snack on board. Quite often the highly loaded routes that have faced the chop are city break or eastern European routes, where people want a cheap fare and don't buy the extras.
The only possible FR routes I can see duplicating BRS is ACE and maybe LPA and IBZ, both served by EZY and FR. EZY don't serve MLA so FR would potentially be diluting their own yields in BRS to MLA.
The bonus to these flights is that they aren't operated by CWL based aircraft and no associated base costs. This is why I can't see FR basing at CWL until they prove a few more routes can work.