The title is self explanatory. Why are new D/EMUs favoured so strongly over a loco set? Surely it can't just be the acceleration advantage. Considering the shortage of (especially) diesel traction in various regions, I'm surprised there isn't more demand for the spared HSTs/225s, or indeed new locomotives and repurposed Mk3 carriages.
I'd also think they'd be cheaper than a multiple unit set as well. Is it to do with efficiency perhaps?
Reliability and track access charges.
Lose an engine on a LHCS set, and it's sat, dead, waiting for assistance, lose an engine on an HST set, and it'll do 100mph at a push, allowing some recovery of service. Lose an engine on a DMU and it'll be anything from a minor inconvenience to a bit of a problem.
EMU stock, particularly the new generation, have duplicate systems - two or three transformers, two pantographs, and form half sets, so one half can still work, the loss of a pantograph isn't an issue, that sort of thing.
Track access charges are a major issue too, because of the sliding scale concerning axle loads, EMUs and DMUs, being only a little heavier than an unpowered coach, are only pennies more per mile than an unpowered coach, when you put all the weight into one locomotive, it becomes significantly more expensive per mile.
Train length is another - adding a locomotive takes up 20m of train length, where you're compromised by signal, platform length and points, you might need that 20m length to fit in a passenger carrying vehicle with 50 seats.
There's also power/braking issues. It's easier to get 4MW down when you've got a dozen powering axles, instead of just four. It's easier to brake and recover energy when you've got a dozen axles with motor generators on them.