Some new hires have lots of 121 experience and could easily go right to a wide body. Some not. It is hard to learn to be an airline pilot getting 1 leg every 3 months on reserve. Or less.
Since we are in a seniority based system, no one gets to choose who goes where so you have to dumb it down to the lowest common demoninator. It is better all new hires go to a smaller jet so they get some legs under their belt and learn to fly for United, even if some don't need it.