Could be a variety of reasons.
For one, it's a good way to get your name out there. And frankly, it's a great idea, even if you're getting a bunch of worthless coins (hopefully not though!)
Also, they're getting more popular because people are posting online about them more often.
Why? Referrals!
It's mainly because there are a lot of cryptocurrency out there that are starting their own coins and they want to advertise and market it to the people.
Yes, that`s true. But unfortunately, another reason why there are so many Airdrops because some of them are scams. When people do their job and get nothing that is sad.