1) You only need one other person to get a high-intensity workout. This is much easier than reliably mustering 22 men for a soccer game, or 10 for basketball.

2) It's a medium-impact, mostly-non-contact sport that doesn't involve jumping, but does involve a lot of lunging motions. This means it causes waaaaay fewer injuries than e.g. basketball (esp. serious knee injuries), and also promotes flexibility in the hips and knees. That flexibility turns out to be important for general well-being and mobility as you get older.

3) You don't need a lot of gear, just the gym shoes and gym clothes you already have, plus a racquet and a 2 dollar ball. You can get a 20 dollar racquet or a 800 dollar racquet, depending on if you're a cheapskate or a gear nerd, or anything in between.

4) You can play it indoors, with or without the cooperation of the weather.