I think with BB especially once we settle on someone we're "barracking" for, that's it, and we're pretty good at viewing all the actions in the house through that lens. If you're Team Tahan she can do no wrong and any possible thing Mikkayla says or does will be spun negatively, reverse that if you're Team Mikkayla (or Tim and Ed, or whoever).
I always bring BB back to politics and here it fits really well. The number of genuine swinging voters is pretty small. Everyone else either picked a side through silent intellectual introspection (unlikely), absorbed it from their friends (possible), or either absorbed it from their parents or picked it in protest to their parents (likely). It's hard once you're a rusted on voter to switch sides - or if you do, you do it once during a life changing period in your life. From then on, the other side is garbage and it would take an apocalyptic event for you to vote for them. (I know, it's a huge generalisation, there obviosuly are swinging voters out there who carefully consider the situation each election, but generally most people at some point self-identify as liberal or conservative and, like a football team, that's it until death).
Now, the question of why is an interesting one for me. I think I might have said this in another thread, but I think the show is not so much a popularity contest as it is a personality contest (though same thing, to a degree). You pick the person whose outlook on life and values and reaction to stress you most admire, you promote character attribute X by rewarding someone else with that attribute with money, via Big Brother, hehe. And it usually comes down to the classic divide of the volatile, confrontational personality who is always willing to go at it, who you always know where you stand with but can be a real rollercoaster of drama, versus the more steady, passive, willing to let things go but also willing to let things fester type, with whom friendship is easier and more reliable but you're not always 100 per cent sure where things stand between you.
Hopefully that was a somewhat unbiased description! And I'm undoubtedly a Type B, so if you think I put that in a negative light I'm only bashing myself! In honesty I think what you need is some sort of balance between the two.