It was always there, always ready to be used by violent criminals, maybe not as widespread as today, although once the death penalty was ended it appeared to become more prevelant.
Not that I am entirely in favour of the death penalty>
However I believe it to be have been brought to the fore more due to a wider news information service, television , internet, etc`. that was not as available in earlier dacades.
Also the cheap imitation guns that flooded into the UK were made in such away that quite a few types could be reworked to actually fire.
The rise in drug related crime and criminal gangs has also made it a greater problem.