Because it isn't about any 'quantity'. League MVPs tend to be based on that, because how else do you choose between players on different teams?
Team MVPs, however, can (not always) be based on how important a player is to a team's success. It doesn't have to be about numbers, or flash. There are ways to contribute other than on the scoresheet. A player that some may consider a scrub that doesn't score much or do anything fancy could be a team MVP because they are a player that can always be counted on to do the little things the team needs to win.
To me, Ellis has made a huge difference for the Canes this season. He hasn't played as much as Ward, but he has helped hold the team together when they needed him to. In the beginning, Ward completely sucked and was not ready to start the season. The team leaned on Ellis to help as Ward worked his way back into playing condition. Now the same with Ward out injured.
No, he hasn't won anything like a Cup, or any trophies, but again where would the Canes be without Ellis? They could have collapsed at the start of the season and already be looking forward to only a top draft pick at this point. When Ward went down, the team could have just given up and written off the rest of the season. Instead the team has continued to play at the same level they did before. Some might even argue better.
It's not much different than arguing that Semin would be MVP over Staal because he makes Staal a better player. Well, that's for one line. Ellis has made the entire team better.
Now, I'm sure many here will disagree with me, but to each their own. I think that's part of why the team is as successful as it is so far that there are several different players being talked about as being the MVP so far.