I almost got into ham back in the early 60s , but my folks handed me a Heathkit catalog just before sending me off the military boarding school - in 3rd grade .
Nobody there was a ham , so I forgot all about it , till about 8 yrs. ago when a friend was at our local coffee shop and had an HT , a hand-held 2m , 70cm .
I asked what he was doing and he mention it was a ham radio , next thing I know I'm off to classes & testing .
As far as the original question , there are many answers , 1st off SLJ you left and are part of the answer .
But like any other subject , there is never only one answer to almost any question .
Recently I saw a stat. that showed about 750,000 ham lic. in the US and more every weekend .
Another large % answer , I think , is that for again a lot of reasons , there are fewer technical / electrical minded out there .
With the miniaturization of gear , its a lot to tackel to make a kit & build your own rig these days .
So that is 3 combined reasons , less inclined young people , harder to build - and a company needs to sell many kits in order to exist .
And touched on earlier , almost anyone is more occupied these days [ even if its just sitting in front of the TV { not a TV in every room back in the 50s & 60s } and sitting infront of the computer screen .
I as a ham think there is no relationship to cells phones , that is a non-ham mind talking .
To me I get on the air and its an adventure , I never know who I may run into on air .
Its a searching for signals thing , a cell phone needs a number to call - no comparison at all , either in technology or mind-set .
To hams have ego stuff going on - when all else fails hams will save the world , attracts many .
Some of those do a good job of being ready , but it appears many are marginally prepared , most lacking in knowledge , training and gear .
With no-code lic. these days it makes it easier to get into the hobby with out any real technical knowledge , then they seem to get lost and drop out .
One the local 2m , 70cm repeaters , things are much slower also , it used to be you had to wait in line to get on many of them about 6-8 yrs. ago .
This seems to me is a good example of all of this , because this is where almost all new hams start and with so many out there it should be more crowded than ever but its not ?
Then there the economy or the lack of , like any hobby , its not cheap and can get very expensive vary fast .
Then there is propagation , with an 11+ yr. sun cycle [ and we are at an unusual low the last couple yrs. ] its harder to make contacts .
Related to something earlier above , tech. learning , at least from a hams perspective learning electronics at the component level and systems , the education is not there anymore .
I have to go back to about the 80s to find helpful books on learning .
I just picked up a copy of The Art of Electronics , by Horowitz & Hill , most sources were in the $100 + range , but I kept looking and found a good condition for about $14 including shipping