In the realm of trains, diesel is a huge economic improvement, so it made the transition easier. Steam locomotives use water and coal at a prodigious rate. Water that has to be resupplied often. A small switcher uses about 75 pounds of coal and 75 gallons of water per mile; a UP Big Boy about 1000 lb of coal and 500 gallons of water per mile. Larger locos were more efficient, but still necessitated a water tower every 40-60 miles. And coal, being a solid fuel, was more laborious to handle.

Getting a large steam engine ready to go from stone cold is often 8 hours, mostly to get the water hot. So it's more practical to just keep them running, don't let them cool down. Combined with the water and coal requirements, steam works best when it's used in a large steam ecosystem. Starting a diesel is trivial by comparison and takes less than a half hour, even with safety checks and such. And no need for water towers. Once the transition to Diesel began, Steam's support infrastructure eroded, hastening its demise.