It wasn't on the top of the shelf because it was a prototype and Steve was about to go into combat missions across of Europe. You don't send someone out on a live mission with just a prototype.
He pulls up a cart with A NUMBER OF SHIELDS, some built, some half-finished (including the one from Iron Man 2).
HOWARD STARK: (CONT’D) I took the liberty of coming up with a few options. (picking one up) This one’s fun. It’s fitted with transistorized relays.
Steve pulls out A PLAIN, ROUND SHIELD from the bottom shelf. He spins it between his palms. It’s light, balanced. Steve pings the simple shield. IT RINGS LIKE A BELL.
STEVE: What about this one?
HOWARD STARK: Oh, that’s just a prototype. Now this one-
STEVE: What’s it made of?
HOWARD STARK: Vibranium. Stronger than steel and a third of the weight.
Steve slides the shield onto his arm.
HOWARD STARK: (CONT’D) It’s completely vibration absorbent. Should make a bullet feel like a cotton ball.
BEHIND THEM, PEGGY ENTERS THE LAB.
STEVE: How come it’s not standard issue?
HOWARD STARK: It’s the rarest metal on earth. What you’re holding there, that's all all we’ve got.
Captain America: The First Avenger
Now why did he create it into a shield? It's possible it was all created into a disc just to test the properties of Vibranium and that it wasn't actually a shield prototype but just a prototype for the material. At that point they may not have known about how to reforge it or just hadn't gotten around to it. Stark was focusing on a lot of different things at once.
It's also worth noting that if that is all they have got, as Stark claims, then it isn't going to be terribly useful on a large scale. You need to find one main purpose for it and well, I can't think of any immediate better uses for it. Any use would only help one person and then who do you decide to give it to? Seems like a lot of potential is in the material but with so little of it that potential is pretty worthless.