The RAF decided that they wanted Grobs, and directed the PE to go for an essentially 'off the shelf' purchase.
Firstly it wouldn't have matter what gliders were brought, Grobs or K21s, they would be in the same pickle because of the paperwork shortcomings.
Secondly for exactly the same reason that they can't go out and buy 60+ new K21s, they couldn't then. Grob were willing to build them lots of gliders quickly (and all gliders are built - hand-built - to order) whereas Schleicher not. Schleicher are still in business, Grob are not at least no in the aviation world.