ISIS until recently was considered a non serious issue. Sure they did nasty stuff, but until the killings in France it was all inside Syria. The thinking was that Turkey was supporting ISIS as a foil against the Kurds. The Kurds are a militant nomadic people that occupy portions of Syria, Northern Iraq and Southern Turkey.
They were some of the United States closest allies during the War in Iraq and afterwards. We armed them considerably, and enabled them with partial funds of the Iraqi oil production. Ultimately this group seeks a homeland not simply to exist in Southern Turkey, Northern Iraq and Syria. They despite all the promises to the contrary they will eventually seek a homeland or nation of their own.
Turkey sees a Kurdish state or country as VERY likely to want a piece of Southern Turkey, and is rightly going nuts over the prospect of losing territorial integrity. The want the US to stop funding the Kurds, and even if we did their share of Iraqi oil funds is somewhat irreversible at this point so they will continue to be armed danger to Turkish territorial integrity.
Turkish funding of ISIS was an open secret between the regional powers in the Middle East, and no one gave a real damn as while they were nasty their primary targets were ethnic Kurds and Syrians not towing their extremist line.
The thinking was that the US could not abandon it's allies the Kurds, per the Turkish demands or else it would be nearly impossible to secure local support in any future endeavor as we would have a reputation of turning our backs on the small guys once they served their purpose.
So why did the U.S. not call Turkey out on their support of ISIS despite it being common knowlege. Turkey has been a historical ally of the United States, they were so willing to go to bat to help the United States they hosted some of our nuclear missles during the 60's. This would have made them a direct target for Soviet nuclear retaliation. Simply put they were willing to take a nuclear strike to help us out.
Turkey's location against the black sea, is an important location as a bulwark against Russian aggression. Russia has a history of taking anything not locked down. Not even two years ago in the Ukraine Russia caused a civil war, and annexed Crimea. In 2008 Russia ran rough shod over all over the country of Georgia.
The view that a strong ally in the region to deter an armed conflict with Russia was the consensus of the state department, and presidents (and presidential candidates of both parties). Romney said Russia was our greatest geopolitical foe, and Obama called ISIS junior varsity.
The thinking has been that Al Qaeda which wanted to target us directly their best day ever resulted in 3,000 casualties. Vs ISIS which hadn't directly targeted us, and to be fair we really hadn't done much to target them. The much lauded rescue mission to strike against ISIS and rescue the Yazidi's was basically a tactical retreat. We sent our airforce into the area, along with Kurdish militia not to attack ISIS but to evacuate an Ethnic group ISIS was cleansing, and we went back to pretty much treating ISIS like a regional hornet's nest. They harassed and irritated people we didnt like - Assad and other rival factions. The people ISIS struck we did like were the Kurds, whom we armed, for their loyalty to us, and whom we couldn't easily defund (They now get a good portion of Iraqi oil proceeds) even if we wanted to betray them.
TL:DR Putin takes anything not locked down. Turkey has strong regional might, considerable historic ally. US/Turkey strongly disagree over support of Kurds. Turkey funds ISIS until we stop supporting the kurds. US and Turkish diplomats shake hands focus on mutual threat of Russia. ISIS in one day kills the same ammount as 1/5 of the people who die in Chicago every year, forcing our hand.