Because DOJ precedent and the OLC memo on the indictment of a sitting President legally precluded him from seeking an indictment or filing charges.
This is all explained in detail in Mueller's report. Pretty obvious you haven't read it, and since you haven't, you're simply regurgitating the spin you consumed through a gazillion "Trump was vindicated" op-eds and Fox News segments.
The OLC memo was not considered in his making his report.