Thats because at the time,back in the early-mid 2000s,iran had no choice but to use/repurpose whatever existing tech it had access to,ie late 40s rocket engine tech,which used less efficient fuels,for the first stage and mid 60s engine tech for the second stage,except that the engines used for the 2nd stage were originally small vernier engines that had to be both modified and then pushed far beyond their original limits,naturally this meant reliability suffered badly.This photo highlights the problem with ISA SLV program.
Simorgh size “class” SLVs should be putting 700-1250kg payloads into LEO if you look at competitors in same size class, but Simorgh is only doing 150KG.
Even much smaller SLV are doing 3-4x Simorgh.
Points to weak/outdated engines or fuel mixture or both.
My own hope is that iran has acquired the paktusan/rd-250 engine from the dprk,as this would give iran the potential to build boosters with the ability to put multi-ton payloads in leo.