Since acquisition MAG have championed & lobbied for updated/faster train services to London & make & continue to make a very valid business case.
Caught up with Government malaise as always concerning any addition & any updates generally to national infrastructure.
One physicality to overcome on the airport site itself in terms of frequency at least is the single track rail tunnel under the runway into the terminal.
I believe it is probably at maximum usage now.
The quick answer is a second tunnel....however.
Stansted Express has the capability to use 12 carriage trains.