But once a patch is made available, it provides an opportunity to reverse engineer the patch to determine what the defect was. This gives hackers the opportunity to devise attacks that leverage the vulnerability.
So you see what happens: if Microsoft releases too frequently it creates a large window of opportunity for infrastructures to remain unpatched and therefore vulnerable to exploits.
Microsoft chose a monthly release strategy as a balance between too often and not often enough. It's not perfect but the alternative is far less desirable.