Dental implants can take 9 months. during this time some other crown or root canal problem happens. not good. why so long for bone graft to get solid?

Bone grafts: It is what it is. Bone integration takes the body 4-6 months at least for enough integration to minimize chance of failure. Bone grafting around implant takes less but sufficient time needed. Other conditions such as irreversible inflammation of nerve of tooth, decay just don't take as long. Other procedures such as RCT or crowns take 1-2 visits generally. See comment below:

Depends on each case: Normally the healing process takes only few months 2-4, but you can get dental treatment on other teeth in the meantime

Your Body's the Boss: Unfortunately, your bone will only heal as fast as it can heal. And, bone heals very slowly. We don't have a way to speed up this process yet. Your skin will heal within a few weeks. A bruise will heal in a few weeks. But bone just heals very slowly. If you break your leg, you'll wear a cast 6-8 weeks and limp for months afterwards...same thing. That's just the way our bone physiology works.