Bitcoin is banned by government because, government feel threaten and they don't have control over bitcoin, government feel bitcoin is illegal because it not a register company, no tax, so they feel is illegal, and banned it.
Few demerits which govt or the organized body consider is that : -
1) It can be misused easily and it can cause a national threat .
2) Black money can be easily rotated and deposited anywhere in the world through this means.
3) Its is traceable but not fully in direct control of the govt like how banks accounts are.
4) Misappropriate in the taxes can happen through this form.