Iran and Pakistan share a long border. It is much easier for Iran to ignite troubles in Pak, than for Syrian rebels to do so to Russia. It is not a good idea to sell weapons to a friendly neighbour's enemy.Russia is selling $hit load of Weapons to the Syrian Regime and no nothing happened to them.
If Pak is really looking at it as an export opportunity, as many pakistanis have been trying to paint it, well there are many other states and non state actors to sell to, from Mexican or Colombian cartels to war torn african nations. Why sell to belligerents so close to you?
Especially at a time when Pakistan is about to go to war on its west - would you want Iran clandestinely supplying heavy weapons to rebels in balochistan and waziristan, and see your armored columns get blown up by them, or your helicopters shot down? Wouldd you then accept Iran's justification that they were simply doing business and looking to boost their defence industry?