I would imagine that all the diesel exhaust confined to the tunnel was the reason that they use electric engines.
And speed, the trains do 186mph it's difficult to make a diesel that powerfull.
After the melting snow that caused the fault the screw up was the usual management SNAFU.
The tunnel, the track on the English side, track on the French side, the trains, and the signalling are all owned and operated by separate companies.
So the first train got stuck nobody told the French operator to stop sending the other 5 trains in - the signalling system can handle 50 trains in the tunnel at once.
Then the first train operator leased a diesel from another company to tow the first train, but instead of just pulling it out of the tunnel into the station they towed it to London (it's final destination) - unfortunately that took 4hours before it could get back for the next train ....
Back when the railways were an inefficent nationalized dinosaur it would have been easy - but now that everybody working there is in competition with everyone else it's chaos as soon as anything out of the ordinary happens.