Why do apps stop supporting older Android versions after some time?

My mother had a shopping app on her old smartphone which runs Android v6. Recently I noticed that the app on her phone was no longer supported. I've seen such things happen on old phones generally with many apps. I just wanted to know, why do companies drop their support to older OS versions after few years? Does it cost them a lot to continue the support to older OS versions as well? Does it require a whole team to be hired to just keep managing the support for older OS? Just being curious about this.