I see no point in the characters being animals, apart from an obvious cash-in of the success of Zootopia. They could easily be animated human characters and nothing would change.
At least Zootopia actually used the fact that the characters we're animals to tie in with the plot.
Animals are much easily to remember as characters, unlike dozens of look a like humans. Especially for kids, with so much characters.
Plus it's their cartoon. Not yours. Then you make your cartoon, then use humans.
Are you saying all humans look alike?!
Why you evil speciest! I'll bet you're one of those evil white sheep!
😜
They didn't need to be animals but the creators decided that the wanted animals. Zootopia didn't need to be animals either, it could easily have been adapted to have been something else.
There doesn't have to be a point. It just is. Not everything needs a reason to be beyond it being what it's creator desired. You and I didn't need to post, but we both did. 
Animals and other creature tropes tend to appeal to kids I figure was their main basis for this decision.
shareOf course they needed to be animals. Name one element of this film, from plot, to score, to character development that would generate anything but bored groans EXCEPT THAT IT'S ANIMALS. Yes, audiences are that easily diverted. And let's not forget how easy this will be to dub for international audiences with no ethnic bias.
Sing is nothing more than a year end money grab which will be financially successful and as quickly forgotten as the songs fall out of public conscious.
It's not what a movie is about, it's how it is about it.
RIP Roger Ebert
I doubt the songs will "fall out of public conscious" since a lot of them have been popular for 30 years or more.
Why watch Idiocracy when you have a TV and the internet?
I am judging from the clips, previews, and trailer I have seen. It features songs by Katy Perry & Taylor Swift, but I see they have wedged in some Queen, Spencer Davis Group, and even Frank Sinatra songs.
Still, it's not the basis for a memorable movie. It's a cute trick to have animated animals imitate the international obsession with musical competition shows. An insubstantial trick that will nevertheless make a lot of money, because as I have originally said, it will translate across cultures and languages well.
It's not what a movie is about, it's how it is about it.
RIP Roger Ebert
It seems to be attempting a retreading of "Cats Don't Dance", but with modern music and no charm at all.
shareIronically, Cats Don't Dance is Sing meets Zootopia. That film was ahead of its time.
Want three steaks?... My mistake. Four steaks. 
And with fewer species involved and restraining the core message, CDD had a much better-crafted social metaphor, I think.
The best-fit would be a society of dogs where the Great Danes look down upon the Chihuahuas. Which, to be fair, the Great Danes can't help doing since they're so huge. 😉