Bi Polar is a mental illness just like addictions are and I agree, bi polar is AS damaging as addicitons.
And YES, people should get through hard times together. Sometimes "together" can be together in spirit and apart physically.
You already made a great start by helping yourself in order to help him "together", you came here to talk.
If you want to continue the road to recovery with him together, get to al anon and you will have your "whys' answered.
Not everyone has the same level or strength, you were able to change not having to leave maybe because you had more strength, maybe he can't
We are all different.
The KEY to this crazy ride of addiction is to know, and seriously know, NOTHING they do it personal.
Please keep posting and reading.