BTEC was necessary due to the ongoing collapse of the old system as mass industrial employment in nationalised or homogenous industries vanished.
Civils has always had a clearly defined structure, underpinned by local authority employers who provide the critical mass of candidates that sustain it. There is no legal equivalent of Mine Manager etc, so that constituency never existed.
The old HND etc system was a parallel system of candidates combining workplace experience and academic training. You didn't need A Levels to enter it, but it was a very comprehensive system. That's probably why the original DipCSM wasn't accredited, because it lacked the overall structure; it was probably obsolescent as introduced.
IMM was always different, because it's membership was international with a wide range of differing backgrounds. I didn't follow the various politics too closely, but it was "received wisdom" in my CSM days that IMinE was basically a manifestation of the NCB's "not invented here" mentality.
Another issue is that whole categories of engineer developed in the rapidly changing offshore oilfields during the 1970s and 1980s. The American institutes didn't cater for them, ICE in particular slammed the door in their faces. Some, like IMechE and IEE embraced it; others, not so much.
That's how I became Chartered; because the IoM3, as it had developed by about 2003-04, allowed me to make a combined application based on various mining, pipeline construction and surveying elements. I wasn't the only one, it the first to do so, not by any means, and I suspect that applicants like me will dwindle in number as the new system becomes accepted.
This brings us back to the continued relevance of CSM. The old Mining Qualifucations Board has long been an irrekevance, because it's conditions can't be met. CSM is working with RICS to provide a recognised alternative to Mine Surveyor, which still exists in principle but in practice, is long dead as a workable system.
Someone needs to fill the gap as the last NCB dinosaurs are ushered gently, but firmly into retirement. Someone needs to represent the oilfield tramps.
Things change
plus ca change, plus c'est le meme chose