The trains are no faster today compared to 15 years ago when 150,000 pax regularly flew LHR-MME. Of course this was not all O&D traffic as there would clearly have been some connecting traffic and environmental concerns are clearly higher up the agenda today than they were then.
As oldart says, price is key. For business people wanting to get an early train from Darlington to London for a pre 10am arrival you will be talking £100+ one way in standard class (in normal times), with the trip back (fixed time) costing £50-£100 so £150-£200 for the return journey. A standard open return on the train is well over £300.