Hola Chris

There's no special reason, it's only that we expressed the same thing in two different ways (tener celos or ser celoso/-a both mean "to be jealous", although the first one literally means "to have jealousy") - I'll change it so we use the same expression in both the affirmative and the negative phrase so it doesn't create confusion. So now you have:

Yo creo que María tiene celos. (indicative)

Yo no creo que María tenga celos. (subjunctive)

Saludos

Inma