The most interesting day of the year to look at the top apps in the iPhone App Store is the day after Christmas.

That's because lots of people have activated their iPhones for the first time, and the App Store charts get juiced by people downloading their most-wanted apps.

This year, FitBit won the Christmas App Store rankings.

christmas 2015 app store rankingsScreenshot/Tech Insider

FitBit was the number one app in the App Store rankings as of Saturday morning, which means a lot of people got new FitBit devices Friday and downloaded the companion app to activate them. It also bucks a trend in recent years, which saw social and entertainment apps like Instagram, Facebook, and YouTube dominate the Christmas App Store rankings, likely because even people who didn't get a new iPhone this year got a new FitBit.

The FitBit surge is also bad news for the Apple Watch, which Apple predicted would get a big boost over the holidays. That's likely true, and Apple probably sold a bunch of Apple Watches, but as my colleague Jay Yarow pointed out Friday on Twitter, every FitBit sold means an Apple Watch potentially wasn't sold.

fitbitMike Mozart/Flickr

There are probably a lot of explanations for this. But I suspect price was a big factor. The cheapest FitBit device starts at $60 and goes up to $250. The Apple Watch starts at $350 and goes up to $10,000 or more.

Some Apple fans will say comparing the Apple Watch to the FitBit is unfair. The Apple Watch and FitBit aren't direct competitors. The FitBit only tracks your fitness, while the Apple Watch does that and so much more, like sending you notifications and running mini versions of smartphone apps.

Apple WatchIssei Kato / Reuters

But as I wrote in May shortly after the Apple Watch first launched, the device's biggest drawback is that it does too much. Its interface is difficult to learn and takes a day or more to fully master. The apps run slowly and don't translate well to the smaller screen.

The Apple Watch works best when you ignore a lot of the bells and whistles Apple added and just use it to check the time, your notifications, and track your fitness. Most of the other features are pretty lame. On the other hand, the FitBit does one thing and one thing well: it tracks your fitness. And it does it for hundreds less than the Apple Watch.

So it shouldn't be a big surprise a lot of people picked a FitBit over the Apple watch this Christmas.