Because the crimes committed by Hillary were actually committed with the consent of the sitting president, and going after her is the same as going after Obama. A president does not simply arrest the previous president once he gets into office. Also the people who were committing crimes with Hillary had the power to collapse the dollar and the US economy and start a war between the United States and Russia. Before anyone could be arrested they had to

• Ally with Saudi Arabia and purge it of corruption
• forge an economic oil alliance to gain leverage over the energy economy of Europe and thus leverage over Russia
• make a ceasefire agreement with Russia to prevent nuclear war
• make sure North Korea wasn't going to nuke anyone once their chains are removed just because they're crazy
• get china to agree not to pull the rug out from under our economy
• find some gold so we can move to the gold standard eventually
• reboot the US economy so that we can survive the slow collapse of all the economic bubbles designed to destroy us
• allow Mueller to finish his report so that arresting Hillary can't be called "obstruction of justice"

The last point is the most important. Remember, Hillary and Obama's worst crimes were committed with Russia. I.E. literal "Russian collusion" which means that for Trump to make an arrest for a crime regarding Russian collusion would be for Trump to directly interfere with the exact thing Mueller was given complete jurisdiction over. That is actually the definition of obstruction, and it's the reason the controlled media has been screaming bloody murder about obstruction of justice from the very start. They WANTED Trump to arrest Hillary, because if he did, they would have a million lawyers write a black and white case for his impeachment and then all of their problems would go away.

They knew waiting would suck for everyone, they knew it would be painful, but it had to be done. So in the mean time, Trump had his military intelligence guys create Qanon and do things like protecting certain free-speech web platforms from censorship and DDOS attacks and maybe even bankruptcy. All so that people like you could continue to talk about Hillary's crimes without being censored or called a crazy person, and so that over time more and more people could be made aware of how bad things have gotten so that when Hillary finally is arrested it doesn't become just another political fight, but an actual movement backed by the people.

Waiting was the correct thing to do.