I was wondering if brushing with miswaak allowed (which has a taste to it), why isnt brushing with tooth paste allowed?

It is permissible to make miswak while fasting. To use tooth paste in the state of fasting is Makrooh (disliked). The minimum taste in the miswak is inlike the strong taste in the paste. Furthermore the paste is a solid substance and liquidifies more in the mouth thereby having the potential of going down the throat.

And Allah Ta'ala knows best

Mufti E. Desai