There is actually a simple answer to that Ric. Gate valves were all one could get 30 years ago. When full bore lever valves came out and did a better and cheaper job people stuck back in days of yore insisted on continuing to install them. To compete the prices and quality plummeted so everyone now hates them.
I agree the Pegler versions are superb, but they simply do not belong in a domestic system when full bore isolators are available. ALL gate valves have quite serious flow resistance whereas FB valves have virtually none. GVs suffer from cavitation FB valves dont.
There is not a single domestic install situation I would even consider using a GV these days.