We haven't switched to the metric system because there's no benefit for most people, simple as. The only people who would benefit are those who regularly need to deal in metric already, and children who would no longer need to learn the common imperial conversions. Most adults know the common conversions, and don't need to deal in metric for anything. Meanwhile, those same people would need to relearn how to estimate everything in metric units, which is a huge pain in the ass for them.

When you are trying to advance a measure which brings most people no benefits, and also incurs significant costs on them, it shouldn't be surprising that it's not popular. It's not exactly rocket surgery.