You want a non-ideological reason.
If we nationalise the energy suppliers, then instead of the experts at Centrica, BP, Shell etc. negotiating for supplies of gas, LNG and oil products with the Americans, Arabs and Russians we will have Johnson, Truss, Sunak, Starmer, Reeves and Rayner and their Sir Humphrey's doing the job! Do you think they could do any better? It would also cost about £300bn to nationalise just the three companies mentioned, and I can't begin to imagine the problems trying to nationalise the Spanish, German and French owned suppliers.
The government was forced to nationalise Bulb in November 2021 after it collapsed and there were no suitable offers to take on the business. This has already cost £1.7 billion leading to an increase in domestic bills for all of us.