There is always favoritism in almost everything  we do in life and that's also gambling inclusive and for several reasons people would want to favour a particular  team or sport over another and for me, one of the major reason I always prefer a particular sport over another is the reason you .entitled above, and I also believe that would also be similar  reason with others because it will always be best when one sticks to to what they know especially  when making bets.
Away from choosing  a particular  sport over the other, i think choosing  a team over another is one of the case scenarios as most gamblers are die hard fans of a team and will always bet in favour of the team when the case always arises.
But truth be told that, I basically don't let favoritism bother me when making bets because I always have the consciousness of making profit irrespective of what sport and game I'm betting on.