Why mining is not profitable after halving?
What can be the exact reason?
It CAN still be profitable, but older less efficient gear probably won't be.
Income will be cut in half (give or take difficulty shift) but the COST of the electric to run the miner won't change. Comes down, as usual, to how cheap your electric is - if it's cheap enough, S7s and Avalon 6s will still be (barely) profitable, S9s should be solidly profitable but will lose well over half their profit, and anything older than an S7 will probably turn into a serious moneyLOSER even if it's break-even or a little better right now.
"The halfing" refers to the fact that somewhere in July, block 400,000 *IF* I remember correctly, the block rewards gets cut in half from the current 25BTC per block to 12.5 BTC per block.
THAT is why "the halfing" is going to have a massive negative effect on Bitcoin profitability.
Bitcoin price rise before then MIGHT mitigate the effects enough that even some older gear on low-cost electric will still be profitable, but EVERYONE is going to take a serious hit to their profitability.
Bitmain seems to normally adjust their BTC price once a day - WHEN they aren't on "Dragon boat festival" holiday.