^Because we didn't have BBCAmerica back then. In the Tom Baker days, we only saw it because local PBS stations aired it--usually at some strange hour, like very late on Friday, or in the middle of Saturday afternoon. But now that BBCA is on many, if not most, regular cable services, it's available to a huge audience. It's no longer just a strange kiddie show from England on some odd little channel; it's now a family adventure Sci Fi/Fantasy show on cable. NuWho's popularity has grown steadily and has really hit the heights with Smith. There's also a lot more media attention, especially after Who topped the iTunes list for the first time.
I'm old enough to have watched it as kid on PBS in the 70's, so I remember. Here in Houston, it actually premiered first as part of a four show afternoon block on Channel 2 (NBC). I was too used to Star Trek, so had a hard time getting into it. Then it jumped to PBS on Friday or Saturday airings (I mostly remember late showings on Saturday nights).
As for Who being a "kiddie show from England".....well when I was a kid, I thought of it as a grown ups show, like Star Trek. It wasn't until I had long been an adult that I was told it was a kiddie show.
And of course I factor in all the various media options available to us in the present for Who's increased popularity.