There are a whole plethora of factors in this decision.
1) CWL is more operationally capable for this kind of route due to the Runway size, depending on A/C they intend to use of course.
2) APD may certainly be a factor for the future.
3) The Qatari's have long expressed an interest in investing in UK infrastructure so are keen to establish links with a growing Cardiff CBD which is gaining momentum rather quickly. The attraction of the Cardiff City Region, Metro and it's Universities is another city based factor.
There's many, and i'm sure you all know there's many more too. Great news for CWL, so can the usual mass complainants stop moaning, live with it and crawl back under your holes. Thank you. Some people just can't stand the thought that CWL has gained a great airline and a great route over BRS and many other UK airports.