No, it's because they can't spare support resources for other browsers. If they know the flaws they have to support workarounds for, they keep support to a man

No, it's because they can't spare support resources for other browsers. If they know the flaws they have to support workarounds for, they keep support to a manageable level. They could, of course, just hire more support staff, but then their productivity would drop and the CEO would have to shave five feet off the length of his next yacht.