Why Mahout doesn't yet have Linear Regression
I don't know if there's a "why" to things like this. It just doesn't exist.
However I think it's the opposite of what you suppose; it's too "easy". Unless you're solving a solution of a ten million equations, it's probably not of a scale that Hadoop is called for. There are plenty of existing packages that can do this really well on one machine. If you want something also in Java from Apache just look at Commons Math for example.
Not to say there couldn't be a fine non-distributed version in the project, but since the emphasis is mostly big-scale and Hadoop, that's probably "why".