why is my foot fungus coming back?

Because your feet are still very inviting as moist warm places between the toes.
And because, possibly, you haven't bleached your tub or shower floor well enough, or you go to a gym where others are giving it to you. And thrown out all your colored socks and bleached all the white ones, and thrown out all shoes that have soft linings that can't be cleaned and sprayed.
It is more rare for a fungus to NOT come back, sorry!

Joy is right on. Feet are the wettest, most air blocked organ. Very difficult to get rid of toe fungus.