They haven't known what to do with Ann since the first season. It initially made sense for her to be hanging out at the Parks Dept. since she was trying to get the pit next to her house turned into a park, but as that's become less of an arc in the show, they've just had to keep contriving reasons to keep her around. And most of that just revolved around which member of the government she was (or was not) having a fling with. This season, she mostly existed in the background until they threw her a bone with the pregnancy storyline, which I guess also gave Chris something to do other than sulking and being pathetic.
His character has sort of been the same way. He does well as a background character similar to someone like Creed or Meredith in The Office, but he's too one-note to be in the show that often, and he mostly got stuck with romantic storylines as well - Ann, then Gerry's daughter, then Shauna Malwae-Tweep, then Ann again... I'll miss his character more than Ann's, because he did have a lot of funny moments, but he's a joke that would've run thing eventually anyways - better for him to get out now than when everyone is tired of him. Ann, on the other hand, has felt expendable for some time. I'll miss Leslie's interactions with Ann more than I'll miss Ann herself, and I think a lot of Leslie's enthusiasm for Ann could easily be transferred to Ben and April (and already has in some cases).