Hey can anyone tell me why someone always quits my band when we are really famous? What am I doing wrong?!

They might be dissatisfied with the amount of stuff you have bought them especially if they are ambitious (they tend to want more), or you may have overworked them (did they complain about being tired a lot?) or they may have fallen out badly with other band members. You need to keep an eye on all these things.

My band is like Guns N’ Roses…a constant revolving door of musicians.

and when is the time your band is famous ?! -
usually after quit some time

then i think the hidden time bomb in the config.txt counts down and your band morale goes down…

change the values under following lines if it really annoys you:
//days in the band before cynicism fits it
//cynicisim climb per day after that point
//amount cynicism can affect motivation