They believe that any modern fripperies, such as buttons, are vanity, abd rhey avoid any modern devices - of which buttons and zips are considered two of.
They are selective though. They have modern machinery for milking their cows and washing up the farming equipment. And although they don't own cars they are allowed to accept rides in exchange for money.
I still have an immense amount of respect for them.
Interesting. Surely buttons aren't that modern?
http://www.800padutch.com/amish.shtml
states; "Their shirts fasten with conventional buttons, but their suit coats and vests fasten with hooks and eyes".
So maybe buttons on their suits are just too decadent!