Divorce or separation instruments after 2018. Amounts paid as alimony or separate maintenance payments under a divorce or separation instrument executed after 2018 won't be deductible by the payer. Such amounts also won't be includible in the income of the recipient. The same is true of alimony paid under a divorce or separation instrument executed before 2019 and modified after 2018, if the modification expressly states that the alimony isn't deductible to the payer or includible in the income of the recipient. See IRS Pub 501.
As for why, you will need to ask your congressman/woman.