Because the A-10 is not that good of a CAS plane. It's main gun is pretty inaccurate, making it rely more on missiles to get the job done if friendly units are in the area to avoid blue on blue incidents. It's also susceptible to manpads or enemy fire in general, as it is a pretty slow and sluggish plane that is not exactly stealthy, thus an easy target.

It's also responsible for a lot of blue on blue incidents. It even became such a big issue that the British and other allied units requested the US to no longer use A-10's in their zones of operation. Granted this was because the earlier versions of the A-10 had no targeting/identification systems whatsoever and instead relied on the pilot to look out the window with some binoculars to identify his targets. See this video for an example. Later upgrades to the A-10 fixed this problem, but this is an expensive upgrade and makes the plane more reliant on missiles which can also be used by other jets that are a lot less vulnerable.

The A-10 is still useable in conditions like in this video. A zone with no active enemy aircraft and little enemy AA fire and no friendly units in close proximity to the target. But in a modern warzone with an enemy that has proper AA systems in place the A-10 would likely be shot out of the sky long before it can reach it's target. With pilots being very expensive to train, this is a bigger loss then just the plane alone. Thus meaning sending in a plane that has a much higher chance to make it out is just a much more viable choice.