That's because the pitches in NZ/Eng have progressively gone worse for spinners & become much more seamer friendly since at least 2010/11 onwards -
stats.espncricinfo.com
Only 11 spin bowlers avg under 30 with the ball post 2010, as compared to 25 from 2000-10 which can be contrasted with -
stats.espncricinfo.com
34 spin bowlers avg over 30 with the ball, since 2010 or so, against 25 in the 2000-10 span!
So basically it was 1:1 wrt spin bowlers avg under/over 30 in the noughties as opposed to slightly over 1:3 post 2010 for spinners in the list.
In the age of DRS it can't only be down to "poor spinners" - if anything the pitches have turned a complete joke for spin in NZ & to a much lesser extent in Eng as well.

Bowling records | Test matches | Cricinfo Statsguru | ESPNcricinfo.com
Only 11 spin bowlers avg under 30 with the ball post 2010, as compared to 25 from 2000-10 which can be contrasted with -

Bowling records | Test matches | Cricinfo Statsguru | ESPNcricinfo.com
34 spin bowlers avg over 30 with the ball, since 2010 or so, against 25 in the 2000-10 span!
So basically it was 1:1 wrt spin bowlers avg under/over 30 in the noughties as opposed to slightly over 1:3 post 2010 for spinners in the list.
In the age of DRS it can't only be down to "poor spinners" - if anything the pitches have turned a complete joke for spin in NZ & to a much lesser extent in Eng as well.