It's the old senario. 4 million anglers? No, only about 2 million with quite a few buying several liscences. I did myself during the 80s, having to buy a liscence for fishing different areas, i.e, North West, Yorkshire, Severn Trent, Wales etc.
These days, there are still anglers that buy at least 2 liscences, especially the carp guys. Only 2 rods per liscence but you are allowed to fish up to 4 rods, hence 2 liscences.
On so called commercials, kids have to be accompanied by an adult, H & S, fishing is time comsuming, some parents have little time to spend with their kids and now we are in recession and money tight and only a few free fishing venues available, some of these now devasted by cormorants and fishing difficult as only larger fish left (this has happend at a local lake near me).
It has also been a few years since we had a decent summer country wide.
Other factors have already been stated, instant success and other activities.