Because he didn’t have the chance to fire Fauci, not really. Fauci is a career civil servant surrounded by a huge bubble of protection against firing, not a political appointee. The most Trump could have done is ordered his political appointee to fire Fauci, who would have instantly appealed as is his right under the laws designed to protect people like him. It would have dragged out for years, we’ll be rid of Fauci sooner this way than had Trump tried to fire him and he would have remained in his position anyway. In addition, since Fauci has an unparalleled level of public and political support there would have been a huge backlash against Trump. There still will be by Fauci’s supporters if he is ever called to account - he is the god-like embodiement of “Science” to them...and everything he did was correct in their view.
Thanks for the useful replies.
*lighthearted announcer voice* Well, let’s all get back to waiting until the shooting starts!