Now that is a very good question!
As best as I can remember it would be 1992 that the CSCS Card was introduced. I was freelancing at the time and we got lucky! The Agency I was working through put on a free 2-day course for us that left us holding our very first CSCS Cards!
It wasn’t quite as cheap as it sounds at first sight, though. I lost 2 day’s wages!
Wasn’t much because the last recession was biting and hourly rates had been driven right down! Meant I was a bit short of cash that week, though!
The came in in 2007 as a result of revision to the CDM Regs. That’s basically why there are the two cards;
bureaucratic nonsense and over-kill!
Suddenly job adverts started showing a requirement for both cards for anyone after a site management job!
Now the really down-side for anyone freelancing is money! The 2-day CSCS course will set them back over £300 and the 5-day SMSTS course over £600 – say £1,000, plus the loss of 7 day’s wages. Say £3,000 in total!!! Okay for those with a permanent job whose employers pay for the courses. Nice few days away from work!
As for why both are required by employers, I’ve heard of two plausible explanations:-
1)   The insurance companies are insisting upon them both – (because they don’t know anything about them, except that they exist!)
2)   The HR people are putting the requirement in the job specification – (because they don’t know anything about them, except that they exist!)
Interestingly, it was just last year that the Head of HSE gave the existence of both cards a lambasting! He reckons the SMSTS Card has to go and, I believe, the matter has been referred to Parliament!