Why the Blazer isn't an off road truck

This isn't something that Chevy can do properly with an aftermarket kit. Throwing a skid plate, lift kit and mud tires isn't enough for it to be taken seriously by off road enthusiasts. They would have to design it that way from the beginning. They've chosen the more lucrative segment, where less investments needed to be made in R&D.

I still think its possible to an extent because they've done it in other segments.
Plus it doesn't have to be that hard or involved. Instead, when done just enough to test the waters and see what consumers want, it'll give them a gauge as to how to proceed with future versions. The stage has been set with the Colorado and its ZR2 trim.

For those who see off road capability as a priority it makes more sense to just go with a different model altogether. The AWD system should allow it to traverse off the pavement, but it wasn't purpose-built for that like the K5 models were. So few people actually fully utilize the capability of a given off roader however, that I don't know if it would have made any sense for GM to invest in that kind of platform.

Its safe to say that most people will look to segments like this for off road capability so its only in Chevrolets best interests to fulfill that role with the new Blazer.

Perhaps this is something that GM can consider for the Blazers first update, or who knows what kind of aftermarket solutions may come about. But I agree with @Kayfive that rarely do people in SUV's venture off the beaten path.