I'm willing to bet because the core i3 is a cheaper option that doesn't sacrifice too much performance. It only lowers the clockspeed by about .2GHz and removes turbo. All at 32% less cost. Probably a good thing anyway since intel's naming scheme is so confusing. At least in the prebuilt market, people won't be confused about their core i5's being dual cores.
On that note, why not get a core i3? It's cheaper and although it doesn't have turbo, is a strong performer at less cost.
On a side note, building your own computer is much easier than you think.
If you want, tell me your uses for the computer and I'll look up some models I think would be good.