Why was the WWII RAF fighter joy stick loop design not used widely?
A control stick with a loop at the top was called a "Spade Stick".  Generally only Hurricanes and Spitfires had them. 

The cockpits on these aircraft were small and it was a problem for the control column to get full travel for roll control. This was solved by making the control column rotate at about mid-stick instead of from the the floor. 

Other aircraft didn't use them because the cockpits were large enough to accommodate a normal stick and still provide good leverage and side to side travel.

The loop was there so a pilot could use both hands on the stick to get more lateral force since the effective length of the stick was now much less than a conventional floor mounted control stick.

Pilots found them to be much more comfortable to hold and loop style sticks did also get used some on larger aircraft with conventional floor mounted control sticks.  

Full right aileron