Its the same reason why the US has never shot down a Russian fighter that has literally violated US carrier battle group air spaces, and the same reason why NATO nations don't generally shoot down Russian fighters that violate their air space.
The last thing the world wants is a war between two of the biggest nuclear powers in the world. It doesn't matter how "justified" a reason may be, both sides will almost certainly disagree on the events that occurred, and that can lead to increased hostility and potentially war.
The reason why both nations "behaved" was because they started to cooperate against ISIS, but that relationship has collapsed, with both sides now accusing each other of committing war crimes.
The situation in Syria WAS a stalemate, but things have changed quickly. The Syrian regime, with the help of Russian forces, have quickly gained the upper hand, and have started targeting western backed opposition forces, despite Russia's commitment that it would avoid such a thing. The US, alarmed at Assad's sudden advantage, green lighted Turkey to move into Syria and slow down regime advances. Now, its a race to see whom is the first one to clear out the last few bastions of ISIS strongholds within Syria, and the US fully intends to win this race.
