We have Covenant gear vendor and the Great Vault where you can pick loot. If they added a vendor it wouldnt automatically be gear you wanted either. What then? People would complain that the badge gear sucks! Just as they say the GV sucks now when they get items they dont want and need.
The GV is an ever changing "badge vendor" where you do content and get to pick an item depending on your efforts. In reality it works exactly like a badge vendor.
And the badge vendor in TBC was all catch up. It was always a raid item level below current progress. If you compare it to now, Covenant gear is exactly that. In current game design a badge vendor would not fit in since seasons comes and goes. Remember Azerite vendor? If you wanted to pick your bis from that it took you 24-26 weeks to get all 3 if you did a 15 each week. Bar 8.3 it would mean you got to use it for a month before next season came and you started your regrind. I remenber all the QQ of regrinding the currency, I bet Blizzard do as well.
Why no badge gear? Because we got it in The Great Vault.