...Because the little latches break off the ring. Having made that embarrassing discovery, there was little to be done except order a new Ring-Turn for my Whirlpool microwave. Happily, you had the part in stock, and the one I received is a perfect fit and appears to be a bit more sturdy than the one included with the microwave from the factory.
The fix was to open the box, take the glass turntable out of the microwave, remove the broken part, put the new Ring-Turn in place, put the glass turntable back in place, job done. It has taken much longer to write this up than it did to perform the fix.