Technology shifts are a factor, but I think there's also been an unsung political rivalry between the IRGC and the Iranian military.
IRGC is about fostering loyalty to the establishment, whereas the Iranian military have mostly been orphaned since the fall of the Shah.
Sanctions have been a factor, sure, but when you look at the Iranian Army, Navy, and Air Force, they're denied of any major platform. Even the modern weapon systems that belong to the air and sea domains are largely used by the IRGC (e.g., fast attack crafts).
I think the bulk of Iran's military capabilities will rest with the IRGC in the future. The traditional military will probably disappear and become some type of national guard for internal security operations, IMO.