WHY did George Washington do this to enslaved Africans at this time?

Because he feared that the enslaved Africans would rebel during the war.

Explanation:

This statement referred to the event when George Washington Prohibited enslaved Africans to join the Revolutionary war.

At that time, many of the generals of the revolutionary army actually also slave owners, even though they're not as many as the people in Southern states. George Washington believed that the Enslaved Africans also had resentment toward revolutionaries and He feared that if the enclaved conduct a rebellion during the war against the British Empire, it will create a massive damage to the cause.

So, he prevented Enslaved Africans to join.

congress was not thoroughly prohibiting states from funding religious schools.