Just a natural shift in getting the more athletic and more skilled players into positions where they can have a bigger impact.
1 good guard isn't going to make an OL good.
However, add 1 Justin Smith to just about any DL and that DL will be instantly better.
I also think that's why OL are typically some of the smarter players on the field. Well, according to Wonderlic anyway. Not that it's some gold standard of intelligence but I think it's a decent measurement of rapid recognition and decision making.
OL are often less physically capable than the guys across from them so they have to use more technique and mental aspects in order to stay afloat.