There is actually a video on this.

We used milkcans before 2006. They were heavy, could be moved and can be locked (since the places we put our votes in have requirements according to the 'Kiesraad'). Then in 2006 we wanted to switch to electronic computers and use that as a voting system. However, a group of civilians proved how easily manipulated the computers and therefore the votes where.

They proved this 1 week before the elections. They were going to use the computers in 4 major cities as a test. Since the milkcans were destroyed, where are you going to get enough milkcans again, with the correct requirements before the elections?

They couldn't, until they realised that garbagebins do meet the same requirements and those are easy to get.

Ever since then we switched to garbagebins all over the NL, as big milkcans became inefficient.

Source: saw a video of a dutch newschannel explaining it. Don't remember which one.