Looking through the list of winners, a total of 9 (out of 16) have worked for the Beeb:
Stacey Dooley (BBC3 programmes)
Joe McFadden (Holby City)
Ore Oduba (BBC presenter)
Caroline Flack (Slightly tenuous here, but she did work for the BBC as presenter/host for a number of programmes prior to winning)
Kara Tointon (Eastenders)
Chris Hollins (BBC Breakfast)
Tom Chambers (Holby City)
Jill Halfpenny (Eastenders)
Natasha Kaplinsky (BBC News)
In some cases, the winner has been the celebrity with the lowest score from the judges, but the overall support from the viewers. I'm not an avid fan of the show as such, although I enjoyed Gemma Atkinson's appearance last year

Could it be that the household BBC names are more likely to generate bias in the BBC viewers?
Could it be that the public may give a lot more respect and votes to the celebrities who most probably haven't come from a performing arts background? Dooley, Hollins, Kaplinsky and possibly Oduba tick those boxes?
Could it be that it's better for the Beeb to have a BBC household name as winner, whether for financial or otherwise reasons?
Could it be that I just shouldn't give a fuck?