Why can't I send photos using AIRDROP from my 2011 Mac mini to my iPhone 6 or 2018 iPad?

Why can't I send photos using AIRDROP from my 2011 Mac mini to my iPhone 6 or 2018 iPad? I can send photos from my iPhone 6 to my iPad, but I am unable to send any content from my 2011 Mac mini to any other devices. When I open up AIRDROP on my 2011 Mac mini, no other devices show up so I can send content to them.