Same thing happened to me with a 32GB Sprint Moto X that I just received two days ago. I have already used the Motorola website to unlock a 16GB model bought directly from Sprint, and I also used it to unlock a 32GB Sprint model that I bought from the MotoMaker site a month ago...but this Moto X that came two days ago (bought from MotoMaker during the $150 off promotions) is showing the same "device not eligible for bootloader unlocking" message that is mentioned in the OP. I have read that this has happened to a few folks, and might be due to Motorola not having that specific phone (IMEI) and/or unlocking code added to the system yet, especially if you just received it? (In other words, some who have received that message have later been able to unlock, they just had to wait for some period of time?) If that is the case, I would imagine that it would be more common right now with the flood of Moto X's that they had to crank out after the $150.00 off promotions.
Who knows...hopefully with the passage of time you will be able to unlock eventually.