Summary. Because Israel gains by the instability that ISIS brings to Syria. Israel will fight ISIS only if ISIS becomes close to form a stable government. In fact, Israel actually attacked anti-ISIS forces in the past, and still does so.
This is not specific to Israel, but extends to other nations too. E.g. weapons exporting nations (e.g. USA, Russia, France, etc) benefit from the instability in Middle East, as it means massive profits for them due to increased weaponry sales.
This is not specific to Syria either. E.g. same goes to Yemen, Iraq, Afghanistan, and soon might be more nations.
Nor that it is specific to our time. Groups of weapons-producing humans used to ignite hatred between different tribes in order to increase wars between them, so that they can sell more swords, spears, etc.
Context. There is no good party in such wars. Even the anti-ISIS forces are also bad. This is a mess with selfishly short-sighted parties destroying nations as they fight to maximise their selfish share of resources, irrespective of the global impact of such wars. The result is a mess that is going to be at the loss of everyone in the long run, even those who gained some resources by it in the short run.
Reason. The reason we still have such crab mentality-like issues is, fundamentally, due to the fact that the world is still a type zero civilisation.
Future. The future might be bright if we, one day, manage to become a type one civilisation: a global civilisation with less short-sighted selfishness, as members of the globe share their resources more openly and thoughtfully for the greater good of all. Economically, this seemingly-selfless investment is actually the best long-term selfish investment for everyone. Only win-win relations are profitable, as all win-loses will eventually become lose-loses.
A pre-requisite of becoming a global civilisation, is to realise global challenges that threaten all of us, which is already happening (e.g. global warming, extraterrestrial threats to Earth, etc). So we're certainly on track. But, we have challenges that may take us back to the stone age, such as nationalism, tribalism, racism, sexism, etc.
History. To be fair, there was a time where tribalism, nationalism, etc, were actually good. That time was when we were not smart enough to see the bigger global challenges (e.g. global warming).
The reason those were good back then is because they were our main tool to push each other to evolve to become smarter. If we hadn't imposed such challenges on each other, we were going to be stuck at barely being smarter than other animals (e.g. lions, bears, etc), which is not smart enough to realise the real problem (global challenges).
In the grand scheme of things, such past wars functioned similar to how the little fights between siblings in a single family makes them stronger at handling bigger challenges outside their house.
As we pushed each other to become smarter (by imposing challenges on each other), we started to realise bigger and bigger challenges, until we met the worst of all challenges: laws of physics that define the count-down clock to the extinction of all life forms.
News. Now that we are here, knowing the bigger enemy, having wars between us is suicidal (similar to siblings that fight each other at a time when they're being invaded by a stranger enemy). We must unite our efforts to advance science at the fastest possible rate so that we survive the global challenges.
The good news: In the grand scheme of things, the grand child of a winged hussar, and the grand child of a janissary, are destined to realise that they are nothing but siblings that used to fight in their parents' house in order to sharpen each other to see the real enemy.
The bad news: The real enemy is much worse: Global and extraterrestrial threats against the existence of every life form that we know of. Today, we must optimise all our resource spendings towards maximum scientific output like no other time in our history. We must push ourselves hardest possible, because we have no idea how much seconds we have left before it is too late.
Responsibility. We are the smartest animals we know of, and we use our resources to consume other animals, plants, insects, etc. If we waste our resources in silly things (e.g. wars, luxury, etc), then we have essentially put those animals and plants down, and wasted their lives in vain.
So, our responsibility is even beyond humanity. We are also responsible for those animals, plants, insects, etc that helped us reach where we are today. If we waste our resources in vain, then we have wasted the lives of other animals, plants, insects, etc, that helped us too.