I would imagine because Iran has priorities.
Right now Iran does not have unlimited resources.
The highest priorities right now are defense, in terms of missile defense, air, space, cyberpace, land, water, and intelligence &c..
Not projecting force ATM.
For every dollar that goes into a shiny new jet that won't see action, is one dollar less to spend on WMD deterrent and SAM. Priorities can overwhelmingly decide the outcome of any 'big'-war.
Iran should overwhelmingly invest in R&D for mass-production of AA battery as well as nuclear weapon tech. and ballistic missile tech. to mount the warheads. Secondly, intelligence and cyber security. Third priority would be the ability for Iran to project force into Persian Gulf, take out enemy ships and mine the gulf...
Lastly, land and air force.
Surveillance satellites, and perhaps someday kamikaze satellites to take out the enemy's, in LEO will also become something of a priority in the future.