If Hamas is advocating for that, why haven't they held a single election since 2006

You have to ask Palestinian Authority (PA) for that, not Hamas. Hamas has won both Gaza and the West Bank, but PA refuse to relinquish it's power and forcefully remain to govern the West Bank.

It was Abu Mazen (Mahmoud Abbas) that delayed and then cancel the election since that. Most probably because of his internal political struggle within Fatah itself, and he's afraid that Hamas will win again and prove the irrelevance of Fatah amongst the Palestinians.

Anyway, the Israel is happy that Abu Mazen stay because they need a weak leader as puppet that govern the brittle West Bank. As long as the weak Fatah remains, Hamas will also remain to be (designated as) a terrorist org, so they won't be able to bring about the cause of Palestinian people to the negotiation table.