Lua is less used than Python in the scientific community, and a lot of the most innovative machine learning researchers already work with C++ and Python. Using yet another language with only marginal benefit increases cognitive load and drains from the researcher's mental innovation budget, forcing the researcher to learn the ins and outs of Lua rather than working on innovative machine learning solutions.
Lua is a nice language. Python 3 is a nice language and there are many new exciting features and development styles (hello async programming?) in the making which will prevent a monoculture from forming in the near term.