Because very often the community is artificially formed due to the Airdrop. The quality of this community leaves much to be desired. A couple of days ago, one project canceled the ICO, because despite the fact of 30,000 people in the telegram group, after the launch of the ICO, only 0.8 ETH were invested in the project. If the idea of the ICO was very good, an Airdrop would help draw investors' attention to the project with a large community, but this was not happen.