Because REME Workshops were nests of sycophancy and backstabbing, where "Serve to Lead" wasn't even given lip service. Most seemed to run on a culture of fear and bullying from the CO down, and none were postings which were generally sought after, which tells a tale. The same remains true of REME Battalions; our officers en bloc (with rare individual exceptions) do not look after their troops.
It gives me no pleasure to run down my Corps in this way, but it is the honest reflection of 30 years' personal experience. The best place to serve in the REME has always been, and remains, attached to another capbadge in an LAD or independently. Sadly, the opportunity for soldiers to do so has been vastly reduced thanks to the self serving reorganisation of the Corps, and they are doomed to spending much of their career poorly led and badly managed in a Battalion, doing minimal trade work. Whatsmore, unless they are someone's pet project / golden child / sport Billy, they will have minimal career guidance or support. Not surprisingly REME has an issue with retention.