only war ravaged cambodia is lower in SE asia and burma is not 'over-pop'ed' compared to neigbours....and i don't think the mil regime was a big factor.Narcotic trade is more profitable than other 'biz' activities might be?  http://data.un.org/CountryProfile.aspx?crName=MYANMAR

Even PPP gdp/capita is only $1600 http://www.indexmundi.com/burma/gdp_per_capita_(ppp).html

http://www.tradingeconomics.com/myanmar/gdp-growth-annual