That's because Republicans have stacked the Supreme Court in their favor. They also control the means to remove the president.

Basically everything that has empowered Trump also works to empower true gun reform from ever occurring.

Any president that tried would not get it through congress. If they tried illegally they would be removed. You might wonder why, and the simple answer is congress. Republicans will not vote for total gun restrictions, that's part of their brand. Many democrats also can't vote for it if they want to retain their seats. A good example is the state of Georgia, it's 2 senators are currently democrats, but the state had leaned heavily republican form years. They barely held their seats. Georgia is also extremely pro-gun and has more relaxed laws than even Florida. If these senators voted for sweeping bans they will lose their seats next cycle. That's not a question but a garuntee. They would likely be promptly replaced.

This current political situation would take decades to change.