Downgrading from Yosemite to Mavericks
It really depends on exactly when your Mac was made. Right down to the week. If the hardware contains things that Mavericks doesn't know about, then it won't run Mavericks, period.

Your model may have been available before Yosemite, but that doesn't mean it's got the same components - just the same specifications. Foxconn might, for example, have run out of Mavericks-compatible USB bus chips but Apple said go ahead with the new chips and a driver will be included in Yosemite.

If you are sure you created your bootable USB correctly (it boots other computers into Mavericks) then you are stuck.