To our 1 day event,
people are sent by their company, to do their technical watch during work time, not free time. Not everyone does tech stuff in their free time or can attend an evening event (it's more complicated for parents for example).

One day events give more visibility to sponsors, create a unique moment in the year to gather everyone (you don't see the same person at every meetups, but at this one day yearly event, everyone is here).
Also, at least in France, attendees are different too.

I think you need a mix of both. Meetups on a monthly basis give the opportunity to keep up to date with what's going on in your tech industry, in your community and help people create strong relationships.