Because he comes in, displaces Brown/Lia/Ward, changes us for the better for a few games then leaves and we have to slot them back in anyway - potentially damaging their confidence + they'll have a lack of game time
If they're good enough they'll get game time - Elliottt will only displace one player.
I assume, as we are between transfer windows, we only had two options for picking up Elliottt -either as an injury replacement or as a guest player.
I'd be very surprised if we don't sign him for the rest of the season when the transfer window opens and I assume that is easier to do from injury replacement status rather than guest player status - some else may know the ins and outs of transfer rules.