Democrats wouldn't vote for permanent tax cuts for the middle class. So Trump did what he could do.
I guess you missed it when Obama and the made Bush's middle-class tax cut permanent, huh?
Budget Deal Makes Permanent 82 Percent of President Bush's Tax Cuts www.cbpp.org
Also, President Obama's "American Taxpayer Relief Act," signed in 2013, cemented a tax cut nearly two times larger than what Trump is proposing.