Slide films are a very small market even within the shrinking market for film in general. Some estimate that slide films are perhaps 5-6% of the entire film market and I have no trouble believing it. It's easy to see why: They're the most expensive films you can buy, and they tend to have short shelf-life. They have low latitude so you need to meter the light carefully or end up over- or underexposing them. They tend to have lower resolution than other film types. The large contrast means they're difficult to scan or print.
Modern negative films are really as good or better in every way, except that a properly treated slide film will give you an exact, repeatable colour rendering every time, and you can see the image right there on the negative. That is the reason magazines and other print have preferred slide film over other media.
But now, of course, digital cameras do it just as well — they have many of the same characteristics, with repeatable colour and high contrast — so the market for slide film, never big, has dropped off a cliff. The movie industry — the part that still haven't switched to digital — uses negative colour film, as do most hobbyists. It is cheaper, has much better latitude and tends to have better resolution. If you are going to scan the film, it's easy to adjust the final colour to your liking today, removing most of the advantage of slides. Kodak recently discontinued its last slide films, leaving Fuji as the sole manufacturer.
With that said, a slide image on a light table (or projected) is a beautiful sight to behold. There is something viscerally satisfying about the almost glowing colors that render even a mundane scene otherworldly beautiful. That glow does not quite translate into the scan or print, unfortunately, but it does give you something to aim for. And shooting slide film is fun if nothing else.
Velvia is really made for nature photography. It pushes muted colours, greens especially, to make them clearer and more vivid. Drab leaves brighten up. Flat, rainy scenes glitter and sparkle. Autumn reds and yellows will pop. Of course, human skin will also "pop" and become ruddy and saturated, and any green tones will get exaggerated. It may give you a healthy outdoorsy look in some people, but make others look like they are drunk or suffer from high blood pressure. Man-made, colourful structures will tend to become overly garish.
For these reasons, Velvia has been considered a bad choice for portraits and street photography. I agree that you want something subtler for portraits, but I think it makes a fine film for street and urban photography. With digital photography, and with "HDR photography" especially, the goal posts have moved. With it-hurts-so-bad-I-must-claw-my-eyes-out hypersaturated images everywhere today, Velvia doesn't seem all that bold or saturated by comparison.
#1 Kodachrome, discounted a couple of years ago, is far more iconic but it was not a modern film by any means, and did not use the same chemistry as today's slide films.