Because he is the only democrat with enough name recognition to even have a chance to challenge Trump. Biden is an old school democrat that will get the older dem votes that an ultra lib left candidate would be too radical to get. They are going to run a younger more liberal black female VP candidate beside him in an attempt to get as many black and millenial female voters to the polls as they can. Once again, just more democratic pandering for votes, after which, they will do absolutely nothing for those constituents or even mention them untill they need their votes again 4 years from November.