Because they're good, that's why. I'm on my third one, because when they eventually fail, they fail spectacularly - no output, and random flashing lights. And then you buy another one, because they work so well! This time around, however, it's had a serious upgrade, and so far it's looking good.
The pitch sensitivity is as accurate as it needs to be to stop you spending the entire gig struggling to get that single green light, without being so forgiving as to be inaccurate. If you're in any doubt as to which tuner to buy, you won't regret buying this one.