I'm interested to know as well.
JR West has been using the same rolling stock from when JNR was privatized in 1987.
If you ask me, I think there are a few reasons for this:
The Kanto region is much wealthier than the Kansai region. Tokyo is where all the head offices, government headquarters, major universities, etc. are. There's a lot of merchants and a lot of entrepreneurs in Osaka, but less "big money" flowing around like in Tokyo.
JR West spent all their money on Shinkansen rolling stock. There are over 200 sets of 700 series and N700 series Shinkansen trains in service, mostly owned by JR West. Because JR Central is financing the Chuo Shinkansen, it means JR East's fiscal burdens are alleviated.
Historical legacy. Tokyo is the capital of Japan, and it was the center of JGR (later JNR) operations. In addition, the country was perpetually at war during the fascist Taisho and Showa periods and nationalized many private lines in Tokyo to more easily mobilize IJA troops. The Utsunomiya Line was originally constructed by Nippon Railway, the Sobu Line was a private line as well, and so on. In Osaka, which has been a center of private enterprise for centuries, private railways led growth. Kintetsu, Keihan, Hankyu, etc. JR West has always struggled to compete in the Kansai region. They wanted to compete with Hankyu for commuters between Takarazuka and Osaka, but couldn't afford to expand capacity or buy more trains, so they instead crammed more trains in the schedule. On Osaka's ancient rail network, delays were common, but delays would mean being subject to nikkin kyoiku. Motormen routinely oversped to not be late, and it was a primary factory in the Fukuchiyama Line disaster in 2005.
Kansai is just weird like that. Osaka is known as Japan's anti-capital. People from Tokyo are reserved and polite; people from Osaka are boisterous and earthy. People from Tokyo speak formal, polite Japanese; people from Osaka speak the informal Kansai-ben. And people in Tokyo get to ride modern trains, while people in Osaka ride the hand-me-down JNR rolling stock from the 1980s.