Possibly one factor. Another factor could be fitness.
2016/2017, 90 minute games played : 1
2015/2016, 90 minute games played : 8
2014/2015, 90 minute games played : 1
2013/2014, 90 minute games played : 3
2012/2013, 90 minute games played : 2
Whether it's fitness or managers not playing him, it's still not a good sign. If he plays in a half decent league, and actually plays a whole season and does well, I'm sure his stock in the NT will rise.