If he’s there at 15, why not just draft him?

I really doubt that the Lions are more interested in Sanchez than they are in Stafford. What’s probably going on is that someone from the Lions organization “leaked” this bit of info to a press member to make Stafford and his agent think that they’re going in a different direction. Since the Lions have the #1 pick, they are in the unique situation of being able to negotiate a contract with their pick before the draft. By doing this, they’re hoping that Stafford will accept less money just to be the #1 pick, as he would still make more than if he were even the #2 pick.
It’s kind of like back in high school when there was this girl (say captain of the cheerleading squad) that you wanted to ask to the prom. To make her more interested in you, you had one of your friends start a rumor that you really wanted to go with her best friend. Once the cheerleader that you really want finds out about this, she’ll instantly see you as a more attractive candidate to go to prom with.
It’s all misinformation at this point leading up to the draft, so don’t take everything you read at face value. I guarantee that in the coming weeks you’ll hear reports of the Lions actually targeting Eugene Monroe when they REALLY want Jason Smith (or vice versa)