While Iraq generally had higher quality weaponry supplied to them Iran had more people in her disposal.

Define "winning", is winning Saddam overthrowing the newly formed Islamic Republic and establishing a puppet government? Is winning straight out annexing Iran (which is quite silly)? Is winning destroying Iran's military (while quite strong before the Revolution most of the top military commanders were either executed or went to exile, so by the time Saddam invaded the Iranian military was quite disorganized?

Officially the reason of the war was to have full sovereignty over the Shat-al-Arab River, an important strategic river which defines part of the Iraq-Iran border and allows shipping, naval and freight access to the Persian Gulf. In 1975 after a stand off between the Shah and Saddam over the river, after Saddam threatened to fully annex the river, an agreement was made to divide the river.

It's sort of hard to explain but Iranians for the most part are quite patriotic for their country and will stand united when facing a threat. So despite the Islamic Republic struggling to obtain absolute control when Saddam invaded the entire country rallies together to push Saddam back. Iranians feared they would be taken over by the Iraqis, the Arabs and for them it was a war for the survival of the country. A bit off topic but it is debatable among historians but many Iranian historians argue that the war solidified the Islamic Republic and its position in power.

While it sounds patriotic and such one major factor was the sheer manpower, similar to the USSR fighting against the Germans in WWII. Soldiers, militia, Revolutionary Guard, even child soldiers, would rush to the frontlines and more or less pushed Saddam back by brute force. One example is where wave after wave of children, with Korans in their hands and plastic keys around their necks (they were told the keys would grant them access to heaven), we're sent to be sacrificed in front of Iraqi machine guns until it ran out of ammunition and was forced to retreat.

By 1982 Iraq was pushed back to the border and Saddam offered a ceasefire with Saudi Arabia offering to pay for the damages inflicted by the war however Khomeini refused, saying he wants to march on Karbala (the Holiest site in Shia Islam apart from Mecca and Medina) and eventually Jerusalem (Al Quds). After 1982 it was pretty much a stalemate at the border with little advance on either side.

So in short, the sheer manpower was a big factor in it. Also don't forget that Iran was also supplied weapons just like Iraq, just not as much (or official in some cases).

That was a much longer answer than you expected but it's not really an easy question to answer without talking about other things. I'm on my mobile now but I'll get some sources, some of which will most likely be in Persian, once I go home.

Quick clarification question: what do you mean by Iraq's superiority in man power?

http://www.meforum.org/441/why-arabs-lose-wars

This is a really interesting article reflecting why Arab armies have been so ineffective in combat. The Iraqi army had some serious problems at every level.

Not to mention the problems the Iranian army had, but you asked specifically about Iraq