Because of the very same reason why Turkey is supporting radicals in Syria. But since Turkey and Iran are mutually dependent on each other (Turkey to Iran's energy and Iran to Turkey as a customer), and since Iran itself is being ruled by a radical zealot bunch, economic ties between the two country is not being effected by these f!lthy policies, hence, once Turkey sort its problem with PKK, it might even become Turkey's proxy against Syria and Iran. There are already rumors that 1500 of PKK militants have gone to Syria to be their problem.
There is this delusion among our Iranian friends here; they tend to compare the intensity of the conflict between PKK and Turkey and between PJAK and Iran. They always brag about how they 'destroyed' Iranian version of PKK aka PJAK within matter of weeks. But i assure you, killing 150 terrorists is not enough to destory these animals. There are other reasons, perhaps secret agreements for why PJAK withdrew from Iran. Turkey has killed over 30.000 terrorists, we've killed thousands of them in just a single operation and it hadn't been enough to destroy them as Iran claims to do just by killing 150 terrorist.
I'd really like to watch these mountain monkeys and Iranian zealots with their minions killing each other, with a popcorn in my hand. I believe, the day is not that far