We tried the "more discipline" thing and here's the weird thing about autistic kids, you can't just "threaten/punish the autism out." More discipline didn't work and was, in fact, making the situation worse. So we got a child psychologist to evaluate our son. She spent three hours in his classroom (observing him but pretending to observe the entire class so he wouldn't act any differently). Then, another day, he went to her office and she talked with him for three hours. She produced a detailed report on our son that diagnosed him as having Asperger's Syndrome/High Functioning Autism. We put supports in place to help him and, surprise surprise, they worked. Whereas before he would have daily hour-long meltdowns in school, he is now having them much less frequently and with much less severity.
My own (admittedly) self-diagnosis came later as we were reading up on Autism to try to better understand our son. I realized that all of these books were describing my own life. I could get a diagnosis, but that would spend money we don't have and wouldn't help either me or my son. So I'm content to remain "undiagnosed" for now.
As far as why are so many being diagnosed now? It's because of better detection, plain and simple. In the past, many with autism were written off as being "shy" or "weird" or (worse) "retarded." (NOTE: Don't use that last word around a parent of a child with autism. I'm only including it as a reference of what was used in the past.) Furthermore, theories of what causes autism have changed. In the past, mothers were blamed. The so-called "refrigerator mom" theory said that moms who weren't loving enough made their kids autistic. This likely kept many from getting a diagnosis as it would be "proof" that they weren't motherly enough. Furthermore, many autistic individuals were simply hidden away and not talked about or referred to as "Crazy Uncle Joe."
Nowadays, better diagnosis, more understanding, and available therapies can help people with autism to function in a neurotypical world. Sadly, we still need to deal with people who, in their ignorance of the true nature of autism, think we should just "be given more discipline" or that we'll "grow out of it" or that we're just "excusing bad behavior."