Current SP stats.
Total number of levels: 8,501
Total attempts: 2,679,699
Average attempts: 315
Total likes: 119,998
Average likes: 14
Total creators: 1,638
Total record holders: 1,933
Picking new levels is not that easy I guess. First, most played means that a lot of players have already played the level and it's likely too easy? Same issue if you go by likes. Most difficult and you have quite a lot of levels with bad luck design or simply too hard for the official SP campaign.
I would guess that Fizzer put some filters in place, then picked the levels. (Say, 5%+ win rate, 500+ plays, sort by most difficult?) He perhaps played with the filter too, using 10/11/12 % win rate. Other factors might be to limit the repeated use of a same map etc.
Last level
https://www.warzone.com/SinglePlayer/Level?ID=11106452nd to last
https://www.warzone.com/SinglePlayer/Level?ID=11374513rd to last
https://www.warzone.com/SinglePlayer/Level?ID=11915954th to last
https://www.warzone.com/SinglePlayer/Level?ID=1193016