] why not talk to the officers, and ask them about Darcy?

Wickham must have said/asked something about Darcy right after the chance meeting in Meryton. When he talks to Elizabeth (at the Philips' card party), he asks where Netherfield is located and how long Darcy had been visiting there. Someone had to have told him where Darcy was staying, and that must have been one of the officers, probably Denny.

But he needed to know what Darcy's standing or reputation was amongst the local people. The other officers might not have been tuned into that aspect of the community, and Wickham decided to ask somebody who would be. I imagine he knew that a woman would be the best person to ask, since they would be most likely to be swayed by Darcy's wealth and estate. I don't know if he chose Elizabeth specifically, but he definitely wanted to have someone local to ask the question(s) he had.