Are you talking about the world or the US? Living in europe, I‘ve never seen a diesel train and the economics are just fine for electric trains.

There are diesel trains in Europe. They tend to be more on branch lines, though, with the main trunks being electric. (Electrifying every branch is not economic.)

In the US, it could still be done (and has), but the economics are not as compelling because the price of diesel is lower.

I'm told that the big railroads in the US have a plan to electrify their main line. It remains a plan because the costs are more high enough that it isn't worth it. If they decide fuel costs will go up by enough they will dust off the plan and electrify.

Diesel-electric trains. They're basically Diesel generators with electric drive motors.