Actually its because this was only the end of his second season in the NFL. So he is an Exclusive Rights Free Agent. Not a Restricted Free Agent. Buffalo and ONLY Buffalo can offer him a deal.
Expect him to sign a long term deal midseason if he improves and continues to hold down the tackle spot.
If not, he'd be a RFA after next year and most likely get a High Tender offer
Despite not being drafted, you can tender him as high as making the compensation a 1st and 3rd round pick