A reddit thread quotes Codemasters community manager Lee Williams explaining that legal hurdles are why there's no modification support in Dirt Rally, and that it's possible this situation will not change (thanks PC Gamer). DSOGaming also notes a comment suggesting that this is not to protect DLC sales for the racing sequel, as they don't envision offering any: "We don’t have any plans for DLC at the moment, it’s unlikely we’ll do any to be honest." Here's word on mod support:

I'd love for us too. Truth is for us to implement it the way people are asking is difficult and sometimes not possible at all.

We couldn't allow people to mod in new cars because of licencing

Even liveries on cars (officially supported) causes licensing issues

Custom tracks - we don't have a tool that would enable you guys to make them.

There are somethings that could be done but it seems that every possible way look at incorporating mod support we hit a legal reason why we can't.

It's not off the cards yet but I really can't promise anything.