Can someone present a coherent argument against legalizing marijuana?

At some point in the near future, we’ll finally legalize marijuana, and we’ll have to live with the fact that we’ve been ruining millions of lives and wasting obscene amounts of money on this absurdity.

While I assumed that almost everyone supports legalization and that the lack of movement in this area is just a matter of enthusiasm and passion, I am apparently wrong. Apparently, just about 50% of people support legalization. This is surprising. Who are these people, and what types of arguments are they using to support their position?