It's important to remember that each nation and indeed group has its own objectives, and often they are conflicting. What's happening isn't the world versus the Islamic State, it's a patchwork quilt of different colours of resistance.
This is a very informative chart produced late 2014. In it we see that even those nations contributing military forces are generally just flying bombing runs, or sending soldiers to train up locals like the Kurds.
The invasion of Iraq has made the west reluctant to start another ground war. You can't hold or secure areas without ground forces; the one thing the western alliance is unwilling to contribute (at best special forces). America and Britain do not want to have to send soldiers in, never mind western nations more reluctant to get involved in middle eastern wars, like France, Germany, Italy.
What complicates this more is that ISIL has spread between Syria and Iraq. This means that victory in one country would not be sufficient, and just as happened in Afghanistan after the invasion of Iraq; there would be a surge after the invaders had left.
Syria and the west are hostile to one another, the west is angry with Syria for human rights abuses, and Syria is angry with the west for the US training Syrian rebels. The Assad regime has historically been friends with Russia. And Russia has been sending forces there to prop up the Syrians in order to keep their influence in the region (they have a naval base in Syria). So if America sent soldiers into Syria they might very well end up fighting Russians and start a major war. The Russians have said as much, even going as far as saying Arab armies must not go into Syria or it'd risk a world war.
So the risks for the western powers are very high, and the rewards very low. The western public likes the idea of destroying ISIL, but they are unwilling to accept what is necessary to achieve victory.
Then you have the context of the Middle East at large, which is even more toxic and complicated. Turkey hates Syria and the Kurds, and so wants to see them both defeated by ISIL; whom their Islamist government perhaps has sympathies with. So even though Turkey could wipe ISIL out single handily, they won't. If they went into Syria the Syrians would freak out and there might be a Syrian-Turkish war. Saudi is also keen to see anyone related to Iran punished; which means they are not that bothered with ISIL; because ISIL threatens Shias in Iraq, Syria (Iran's ally), and Iran's Revolutionary Guards sent to help the Syrian regime.
Syria is even more of a mess, and the government is fighting various rebel factions. Syria and Russia are only concerned with fighting Syrian rebels, in part because ISIL fights with them too. It doesn't make sense to attack the enemy of their enemy until their enemy is dead.
Also worth pointing out is that the Islamic State is infused with thousands of Saddam's elite soldiers. After Saddam was executed his Baathist loyalists had no one they could turn to. They feared for their lives given Shia death squads seeking revenge for abuses suffered under Saddam. And so they turned to the only other Sunnis around, the jihadis; and the unholy alliance of the insurgency became a marriage.
In summary, those capable of deploying the forces to defeat ISIL outright either don't want to (Turkey and Russia), their people don't want them to (the West), they have been threatened not to (Arab nations), or they are struggling to mobilise their forces (Iraq). Others who could contribute don't want to push for total victory over Islamic State because they don't feel the need to secure land outside of their territory (Kurds), or just want to stay out of whole mess and probably "wish both sides the greatest success" (Israel). This means that a relatively small ISIL force is able to do a lot, because of the world's inability to cooperate. Nonetheless, ISIL has been losing ground. So it's not all bad.