Expense. That's one of the first things considered when looking into a sport, there are lots of sports, and they all cost lots of money. Hockey is expensive, you buy gear, lots of gear for your kids and replace it constantly as they grow up, you pay each season, camp or year for them to be on teams, it adds up. Perhaps the best example is skiing, with skiing you buy the gear which costs quite a bit, then each time you go out it costs money, you pay for lift tickets, food and drink at the hill in the middle of nowhere...over and over again all season, which is only about 6 months long each year. If you're adventurous you go backcountry skiing, which takes more gear though costs less to do over and over.
Using the river costs nothing, you simply get on the river, a navigable waterway guaranteed (for the most part) by the law of the sea, though lately they've bent the rules for the entrepreneuring capitalist Independent Power Producers for the reason of corporate profit at the expense of the general population for private enterprise at the loss of navigable water. For the most part you drive up, leave a car at the take out and head to the put in, run a waterway and go pick up the other vehicle. Like skiing without a lift (this is probably the main reason it's not a mainstream sport, which is good or bad depending how you look at it).
There are a few things you NEED to kayak; a kayak, PFD, paddle, spray deck (skirt), helmet...as water gets colder you can add a wetsuit, drytop, drypants, drysuit, fleece liner, headcap...then as you get into more risky water things like a full face guard helmet, elbow/forearm guards and a creek boat for increased safety and buoyancy..or specific equipment for playboating like a playboat and possibly a shorter paddle with less feather if you prefer.
The basics used can start you around 300-500 dollars if you buy used equipment, then as you develop a taste for what you want and where you're headed you might want to upgrade for specific purposes and get specific styles of equipment, also once you are at a high enough level to do so you can demo boats on demo days and compare models against each other, this is always a good idea when considering a new boat.
Geographic location is something to consider as well. If you live near mountains you might pursue creeking or a place with lots of waterfalls. If you've got high volume rivers you might need equipment catering to that, if you live in a flatwater area of the pairies you might want a playboat to go hit that whitewater park in the middle of your city. Another thing to look at is higher quality paddles at this point (hang on to 'em!).
As you progress and get better there are safety items to consider having as well, things like a pin kit, rescue vest, overnight gear, breakdown paddle, properly outfitted creeker with steps to get out of a pinned boat, extra webbing, rope, carribeaners, a larger throwbag or two. Also extra equipment like an SLR camera to take high quality pictures of your runs, a GPS to scout new creeks and all the accessories you'll need for a multi-day river trip like sleeping/bivy bag and food.
The money you spend on the sport will typically equate to the time you devote to it. Getting into it I thought it wouldn't be bad, it does add up and you keep thinking you're done but there's always more gear to get! It doesn't help if you lose or destroy boats and gear!
You'll get books, guidebooks, equipment for scouting canyons, equipment to explore caves along the way, videos...you name it, the possibilities are endless!
Bottom line is when you compare it to other sports, if you keep it manageable, it is one of the best bang for your buck activities out there...and who wouldn't rather do a flatwater playboat session at the lake or park and play surf session on the river or lap a sections a couple times than hit the gym? Much more fun!