Sanctions allow our politicians to convince their constituents (at least enough of them to get elected) that they are doing something while delaying the inevitable, necessary and, unfortunately, costly intervention.
In the meantime they are squandering an opportunity and setting themselves up for trouble in the longer term, if not failure.
If they act when a crisis breaks out and chooses a side then when the intervene they can count of the support of at least part of the population, possibly a majority in some cases, for some period of time. Long enough for the locals to decide if the intervention was a good thing or a bad thing.
On the other hand, if they hide behind sanctions, as they did in Iraq, they will create an entire population that is resentful and has to be won over. At the time that we cross the border the reaction is not "Thank God you're here" but "It's about bl**dy time, what took you so long". They are not going to cut any slack while attempts are made to pick up the pieces and sort out the mess.
Sanctions are an unproductive and cowardly implement of war. The word is diplomatic speak for siege or embargo, both considered justifiable "cases for war", that allow the besieged entity to defend itself by all means at its disposal.
Sometimes such a strategy may be defensible in the short term, buying time until intervention is possible but it can't be a long term solution.
Sanctions, embargo and siege never hurt the ruling classes, seldom hurt the military but always hurt the young, the old and the sick as well, disproportionately, the female. You are not going to make many friends by telling a potential fighter that you had to starve his children, his parents and his wife in order to save him.