I think it is because of the way kids grow up nowdays. In short children today are brought up in a "bubble" and their immune systems are not allowed to develop.
They do not play outside, they also live in houses where the windows are never opened. They spend the vast majority of their early years never exposed to their natural environment and thus never build immunities to pollens and other things in the air. Hell, I grew up in a house with both parents who were heavy smokers, so did the rest of the kids on my block. None of us ever developed Asthma or any other respritory problems. We also spent every waking moment we had when not in school playing outside.
The freaked out, overzealous parents of today have to dunk their children and everything they might come in contact with in Lysol and Purell Hand Sanitizer, thus they do not develop immunities to run of the mill bacteria/viruses that have been around forever. When we were kids we did things like *GASP* drink out of the hose, share a bottle of pop without pouring it into individual glasses and our parents version of "dunking us in Lysol" was making us take a bath before going to bed because we were muddy and dirty from playing outside all day.
When I was a child I had just about every childhood illness around-Mumps, Measles, Chicken Pox, Whooping Cough, etc.. Sure it was no fun and I still have some Chicken Pox scars but I also have a highly developed immune system.
I agree with that theory. I read somewhere that the immune system is like a muscle: it needs exercise to maintain its integrity. Now that may be an old wives' tale but who knows? The problem today though is separating factors that might cause asthma from factors that merely parallel it (there's a technical word for that but it's too early in the morning for my brain to kick in, especially on Fridays when I am mentally incompetent pretty much all day).
Is it pollution, lack of pollution, MTBE, cigarette smoke, too much emphasis on antibiotics and cold medicine, genetics, hyper-hygiene, on and on? Some of those factors could be a cause, some just happening at the same time. Asthma is a real disease as I know from my husband's experience. I only point anecdotally to my own experience in saying that at least three diagnosed cases of asthma really aren't asthma so whether it is an epidemic may be academic.
A couple of years ago, a British doctor (it was a woman, but I forget her name) thought it might be that kids are overimmunized today -- nobody's allowed to get what used the standard childhood diseases (you mention them!) and this leaves their immune systems out of whack.