Cats five pretty well sums up the advantages of the K21 glider; London Gliding Club made this model the mainstay of the fleet and it is still doing good work, but at least one K13 glider is still used for spin training, which in the civilian clubs is considered a vital element for serious glider pilots. Lasham Gliding club still has a large fleet of the K13 glider and considers this to be the best all round training glider ever built, so does the club at Bicester.
Trouble is you can't build a wood and fabric glider using a mould, so the construction costs put it out of consideration. Maintenance takes time as well, but many are still going strong.
Polish gliders had a record of doing exactly what it says on the tin; if mishandled, they would spin. And recover using the correct actions, unfortunately not every pilot took the correct action when called for. The K21 would be a safer choice for ATC.
Certainly fibreglass maintenance has proved not to be beyond the skill of most civilian shops; they can cut and shut a bent glass glider just like new.