Thread 404'd when I'd already typed out this behemoth comment, so fuck it. Mods are fags.
Econ PhD student here. I think there are a couple reasons so few on the extreme left study economics at the graduate level, which is a necessary condition to become an academic economist.
1) Self-selection. I suspect that most people who identify as Marxists or have similar beliefs have an extreme aversion to mainstream economics in the same way that I and most others on this board have an extreme aversion to gender studies. Therefore they choose not to study it at the graduate (or even undergraduate) level opting instead to find another line of work or study since they perceive mainstream economics as incompatible with their ideological beliefs, perhaps at a foundational level.
2) Math. While your undergraduate economics courses might have given you a different impression, academic economics is actually very mathematically rigorous (scroll through some of the articles in Econometrica if you don't believe me)*. I've noticed that people on the extreme left tend to have relatively weak quantitative skills, so I think most bona fide Marxists would have trouble getting into and completing an Econ PhD. You basically need to be a math major with a high GPA (among other things) to get in. How many extreme lefties do you know that fit that bill? The reason why mainstream economics has become so mathematical is another discussion...
3) Network effects. If you spend all your time around academic economists, you'll likely pick up some of their beliefs.
*
http://onlinelibrary.wiley.com/doi/10.1111/ecta.2015.83.issue-1.x/issuetoc 
