I'll probably get downvoted but fuck it - I use disposable plastic K-cups every day, and I do it with gusto. Some days I use styrofoam cups, but most days I use my mug.
The honest answer is convenience. In my opinion, the only way the masses will consistently make the healthy, environmentally-friendly, or otherwise "right" choice is if it's already the most desirable.
that is, if the person only uses disposable K-cups. this wouldn't apply to anyone using it to brew their own coffee without trashing a plastic cup every time.