It's sorta the same psychology for why there were 23 GOP candidates in the 2016 election. "We aren't Obama! We're all white dudes that love money! Vote for one of us!"
In this case, it's "We aren't Trump! We are a diverse group of people that want to fix social issues! Vote for one of us!"
They might not be aware of this, (or they might be, not sure) but basically the subtle message is there. Yeah, it's a no brainer that none of them are Trump. However, if few candidates ran, what subtle message would that send? (Take for instance, the unconscious message that would be sent if only Bernie, Biden, and Warren were running. 'those are all old leather bags of skin!' et al, would be common epithets. The DNC seems to be playing the field, so to speak, and presenting a selection of candidates which correspond to a smattering of biases, hoping that one of them will stick. Party leadership likely wants one of these contenders to steal mindshare from Sanders and Warren, because those two would seriously upset the status quo apple cart.)
Ultimately, the number of candidates will be culled strongly in the primaries, but at this point, making a big show of having lots of "eager, willing" people that want to replace President Toupee sends strong subconscious messages of the sort I just mentioned.