Well let me tack a wack @ this :-)

John, your question, although valid to some degree, sounds strange in this day and age(no offence intended)
it is after all an IT revolution(or evolution if you will) happening outside, right this moment.

Tablet is an evolutionally step for consumption device. Key word is "CONSUMPTION".
Tablet was not designed or intended to replace PC on content creation front.
It was intended to replace an over-powered and oversized devices (read "PC"s/laptops) used to day for simple tasks like read email/browse web/read books/news etc.

Just because some individuals out there try to use Tablet device as a replacement for the full functional PC
And that run screaming that it does not work. Does not mean that Tablet have no place in today's life.

That said,
For the most obvious reason tablet craze is out there is Apple Aggressive Marketing.
Apple have brought the Tablet Computing to the masses and now everyone want to jump on the band wagon regardless or actual need or not.

Even though tablets have existed before few years ago, they were a niche market. Only some enthusiasts and some professional in highly specialized markets had/used them . the devices of the time were too specialized and had too small of support in terms of software and contents to be appealing for general public.

Apple changed that by coming out with an attractive looking device with an UI designed for General public (no computer skill needed to operate) and a good back bone support for content availability.

So as more and more consumers bought into the hype and got the "i-Pad" they discovered that
Many of them do not really need a PC for what they do every day.
That they can use a Portable device instead and it is more convenient faster and easier for them.

And that is the gist of it.

Tablet is more convenient and easier to use for content consumption which most people out there is doing every day.
And for the selected few who also need to create things or do a heavy processing of data, there are still nothing beats the PC and so they might have both, a PC for heavy data/contents processing and a Tablet for more convenient consumption.

Note: This post was edited by its original author to replace with revised posting on 03/01/2013 at 1:33 PM PT