My opinion is we haven't got the players.
Years ago there used to much debate about players who never made the tour, not nowadays.
The players who do go can only play 'Cameo' innings very few can turn 100's into 'Big Hundreds' very few can bat without the need to score runs, frustration sets in and the wicket falls instead of the bowler being frustrated, this in turn leads to bowlers who only have to bowl to take wickets not ones who can winkle out a batsman. Much of the blame for this must lie at the T20 / One Day door,
But a batsman only has very limited opportunities playing 12 County Matches - 24 innings max mostly played in the freezing cold start of season or at the end when a lot of interest has waned where as back in the days of 3 day matches there was the potential for over 100.