Because:
A. Yakupov sucks, let's get that cleared up immediately.
B. The Penguins are already literally overflowing with young wingers, to the point where they might have to lose one (or more) to waivers or just dumping them for picks. Signing Yakupov to a 2 way deal (which doesn't prevent him from going on waivers) doesn't help that problem, especially if you have any inclination of giving him a shot in the NHL before burying him in the AHL.
C. Yakupov has the hockey IQ of a carrot, which makes me think he'd be a terrible fit in this system despite his speed.
D. Signing the bust Yakupov has no upside for the Penguins because even if he somehow manages to piece it all together, the Penguins don't even need him in the first place and he'd be at best our 6th best winger long term (behind Kessel, Hornqvist, Sprong, Sheary and Guentzel). They don't need him on the PP because that's where Crosby and Malkin play and they don't need him at ES because that's where Kessel, Sprong, Rust, Hornqvist and Sheary will spend time playing.