High-grade 'skunk' weed and a few concerned parents.
Under New Labour (not to be confused with the Labour Party) the generic catch-all term 'cannabis' was down-graded to Class C. So still illegal.
Then, after a few complaints from parents with kids ending up in lunatic asylums due to smoking too much high-grade weed, something had to be done.
The market for weed has changed a lot over the years, in the last century the product came from who-knows-where and was generally that resin stuff with who-knows-what in it.
Then the market changed to the regular 'skunk' weed - dried leaves of weed.
This then became a bit of a racket, if you believe the Daily Mail it became the business of Vietnamese gangs with the money being laundered through cash only nail bars. For a while the product was quite dangerously adulterated with wallpaper paste sprayed on to make it look extra good. The slight problem with this is that the wallpaper paste would not do wonders for circulatory systems, giving rise to blood clots etc.
There was also a side-problem of legal highs, e.g. 'spice', these really tipping people over the edge into psychosis.
The adulterated weed was soon rejected by the customers and even stronger 'high grade' weed began to be what was available.
By now the older sticky resin type of weed that your grandfather smoked whilst listening to The Beatles and wearing tie-dye clothing was no longer available. We had gone from 'shandy' to 'absinthe', a different beast.
Also to note is that the government is nowadays Conservative so there is nobody in power wanting to enable people to enjoy the finer grades of weed - just ban it is the attitude. People who do smoke the stuff do end up with ideas above their station and you can't have that.
Often the word 'psychosis' was used to describe the kids that worried their parents by smoking too much of the stuff. 'Paranoid' was also an oft-used word. Sure the more modern weeds are a lot more intoxicating than the cannabis resin that went around in the olden days but it is always easier to blame the coping strategy rather than the root problem, e.g. the difficulty of being a teenager in modern Britain.