Well in my own opinion, the barrier is the language itself. MetaQuotes the maker of MetaTrader is based in Russia, and most of their programmers are Russian. For users they might not feel much discomfort for seeing some text in Russian, but for big brokers, of course, it is a big discomfort, because an acceptable platform for big broker must have wonderful functions as well as wonderful supports. MT has wonder functions, but lacks of wonderful supports. If you write any technical email to MetaQuotes, you'll get reply in weird English or even no reply if the answerer doesn't know how to read/write English. Also, their API, for example, are commented in Russian which is no good for most English developers including brokers. In conclusion, if MetaQuotes was based in USA, MetaTrader would be very popular in real trading now.