The basic reason is because our jaws are smaller than our ancestors' were, leaving less room for the teeth.
In fact, I believe that the statement "in the distant future, our descendants will not have wisdom teeth", is one of the few things we can say with any degree of confidence about human evolution. The reason is that occasionally, even in developed countries, an infected wisdom tooth is fatal before childbearing years.
I'm not sure what you're getting at with tusks.