At the present, there is not much of an incentive. The average user who isn't committed to the site probably doesn't care about reputation or site etiquette. As long as the can keep asking questions, they will be happy. Yes, they could get downvoted but once the question is edited many of their post will need to maintain negative scores for a question ban to kick in.
If the community would be willing to work together, we could have an incentive promoting the use of mathjax to seasoned users (not considering brand new users since their experience with the site norms would be zero) that refuse to comply with etiquette. First, let's consider an example. Suppose you have a neighbor who borrows your tools but never cleans them when he is done. One day a new guy moves down the block so you warn him that Jim over there will borrow tools and never return them clean no matter how many times you have asked. Now, this new guy is abreast of the situation and may not let him borrow any tools. How can we apply this concept to the site?
If we know of a user who has refuse to help the community by learning a little mathjax, we can start warning other users via comment on their post--nothing rude or demeaning just the truth. If then the regular users ban together to stop editing and answering those post, this non-complaint user will have three options $(1)$ learn to conform to the site norms (path of least resistance), $(2)$ use another site, or $(3)$ quit using online help all together, but since many users use online help as a last minute aid for assignments, this probably will not be a realistic option for them. In order for this to work, we would all need to work together on this task since not all of us will be familiar with all users. If I know a user who refuse to work within site norms, I would need to make the comment letting the rest of the community know about this or someone else who isn't familiar will just edit and answer. Eventually, the regular active users will be aware of the major offenders and the site will cease to function how they would like. However, they can regain this functionality by learning mathjax.
Out of the three potential possibilities, $(1)$ and $(2)$ are probably the most likely and $(2)$ requires more work than $(1)$ since not all online math sites have a huge user base so finding one that gives them seemingly instant help may not be an easy task. Additionally, most people dislike change so learning mathjax is the path of least resistance since they wont have to go searching for a new community.