Can the Discovery extend it's warp bubble to tow the Glenn home? or maybe towing the Glenn would reduce the ships speed and shields (extra shield surface area, and tonnage is a drain on power, or/and reduced efectivity) to a point where it was unwise to be attacked by Klingons.
How long would the Discovery have to wait and guard the Glenn, because if it's just left there by itself, the klingons will come back, while it waited for a ship that could tow the Glenn home if the Discovery can't, which would require increasing the size of it's warp bubble by more than 200 percent while using a tractor beam, when the Discovery had really important things to do.
If the crew was "warped" maybe the ship was warped too?
Trojan horse much? Goodness knows what the klingons did to the glenn in the Hours that they had it to themselves.
The ships engines were so extremely top secret, that there was no federation shipyard/drydock that they could take the Glenn to, without there being a terminal security threat.
If Lorca stripped the Glenn for parts, and resources, which is smart, he may not have wanted to explain that to Star fleet command, and any of the other reasons above too.