Because they are not slobs.
I am with you. Even here in the north I am impressed with how nicely church going black families dress for church.
At the A.M.E. churches in my town, the black folks enter into the house of the Lord dressed to the nines, right down to the small children. They are a reproach to the rest of us.
For reasons of piety & decorum, or for other reasons. For working class folks in a past era, dressing for church was a weekly occasion & a chance to set aside one’s drab proletarian garb for respectable finery.
Now, it’s just a matter of convenience, only would it break the bank for us guys to wear slacks, a dress shirt & sport coat in the place of worship?
As for the ladies, my standards are far more liberal.
;^)