Cheap, abundant, available and with guys that had 5 years of combat practice with them... Jet engines were not too powerful at the time and jets themselves were kinda clumsy... that until the MiG-15 entered the fray. The documentation regarding sweptback wings was still translated and tested, so many designs had straight wings... designers still struggled to find an optimal area to place the engines... and those were so wide a jet squad resembled a ballet or overweight girls.
Check for instance, the ME-262, the early Yaks with jet engine and the first MiGs... curiously, the designers ignored the experience of Horten, whose planes had the engines in the fuselage, as modern ships.
Piston engined fighters were also best for strafing and light bombing... and the bombers were also propeller-driven, so the old fighters were kinda appropriate for the escorting task (later assigned to flying gremlins like the F-82, a Frankenstein-esque fighter that in essence was a wing with two P-51 fuselages over it. Curiously (and sadly) the prototypes had the engine of the Mustang but the series production got Allisons, too heavy and underpowered by comparison.... since the Packard engine was the Rolls-Royce Merlin (built under license) the USA had to pay royalties for each engine built.
Jets were marginally faster than fighters and frequently less agile and worst armed... that also changed with the MiG 15, equipped with the most devastating guns ever installed in a fighter: while Mustang, Sabre, Mosquito, Corsair, Spitfire and/or Thunderbolt used almost as one .50 machine guns (and 20 mm cannons here and there), the MiG had two 23 mm cannons... and one 37 mm piece... that kind of firepower is not to shot down a plane... but to vaporize it!
When this ship entered the fray, the prototype of the F-86 was being built with straight wings... at once they redesigned it with sweptback wings... even with inferior guns (MiGs duplicated Sabre's guns range), Sabres were more stable as guns platforms and had pilots fresh from WWII; Soviets had lost their best men against the Germans and Stalin's perennial paranoia send many men, both pilots and designers, to the business end of a rifle squad, something that already had cost him dearly when Germany invaded USSR in 1941.