It isn't surprising that the Wii U would have it's lowest month ever. June has enough games that there will probably be an uptake but aside from that I figure the "lowest month ever" title will switch hands continuously over the rest of the year.
Why would someone buy a Wii U at this point? Their are very few games on the horizon. I'm looking forward to something like TMS but that's a niche title. There won't be that many people for which THAT is the game that pushes them into buying a Wii U. Usually that's the approach, where someone is waiting for enough games that really catch their interest to be released before they end up buying a console. If you haven't been swayed by the games already released, you're not going to be swayed by the few games still to come. All the really big games have already come out with the exception of Zelda. Except that is also an NX title so a Wii U purchase is unnecessary. At the very least you would wait and see what the NX is before making a decision.
The only other thing that can spur sales is a price drop. That can move the line so to speak. Maybe someone was waiting for a few more titles to justify a purchase but with a lower price then they need less titles to justify it so maybe that ends up being enough. You can also get some "eh, what the hell" purchases too.
But with neither of those, the sales are just going to drop more and more. If you feel the current lineup is worth the current price you probably already own a Wii U.