I read somewhere about that and the theory was that this is intentional, a big chunk of his base are functionally illiterate and talking like this makes him just understandable. Add to this the fact that speaking like this makes him sound very nonpatronizing. Finally it's good marketing, it's the same thing Ben Shapiro does in his annoying argument voice. Now that I think about it even Obama has a very easily distinguishable way of speaking.

All that to say, it is a conscious choice imo.