Because Mokona's amazing translation skills are a very amusing plot point. Also, crossing over to Watership Down is always great to pull off, especially if the other series is human-based.
Assume Mokona's close enough but not with them, so that's why they're looking for him by checking rabbit warrens. Yes, it makes sense.
Disclaimer: Don't own any of them, more's the pity.
"Hazel-rah?" Blackberry reported with a dazed look on his face. "There are three Men outside. They told me they want to talk to the Chief Rabbit."
"Men. Talked to you." Hazel repeated, sure that his ears needed some cleaning.
"In Lapine."
"Blackberry, are you sure you don't need to rest for a bit...?"
"Hazel, I'm not delirious. Go outside and see for yourself- Fiver's already talking to them."
And when he exited the warren, sure enough, three Men were stretched out on the grass, conversing rather deeply with his brother.
"So in a sense, you are a dreamwalker?" One with red eyes and fur black as Inlé asked, voice rumbling like a hrududil.
"You could say that." Fiver sat before them contentedly, as if holding a conversation in Lapine with a bunch of elil was as normal as chewing pellets. "But it's more of seeing the future, what will happen, and most of the time I don't really understand it all."
"Hyuu, that sounds really scary." The one with fur the colour of dried grass added. "But have you ever talked to anyone else while you see all this? Sort of within your head, not to someone in the real world."
"Not to other rabbits, at the very least. There was once when I talked to a Man, back when we thought Hazel was dead, but he didn't feel real, so I don't think that's what you're talking about. Maybe I'll do like you said, and try to find your friend while I sleep. What was his name, again?"
"Watanuki." The last one said, brown eyes flicking in his direction. "In any case, I think we've got company, so we should probably wrap up this discussion before his eyes drop out of his head."