I agree with you about Woodley being massively unappealing. The strange thing is his PPV numbers are just as dire as Bisping's. I never understood how a journeyman like Bisping became one of the highest earners and those figures can of compound. It's almost like the UFC has been trying to massively market him despite a lack of interest from the general public. So I'd say GSP chose Bisping because he thought it would be an easier fight and not necessarily a bigger one.
What you are missing is that Woodley isn't a draw at all while Bisping is. Comparing their numbers on PPV is only part of the picture. Here are Woodley's fights as a headliner and some of Bisping's since his career as a headliner is much longer.
Woodley
UFC 214 vs Maia Main Event Cormier Jones 2 PPV 860k
UFC 209 vs Thompson Main Event PPV 300k
UFC 205 vs Thompson 3 Title fights McGregor vs Alvarez Main Event PPV1.3M
UFC 201 vs Lawler Main Event PPV 240k
When on his own Tyron draws 250-300k
Michael Bisping
UFC 204 vs Hendo 290k
UFC 199 vs Rockhold 283k
UFC FN 84 vs Anderson Fight Pass event in London
UFC FN 72 vs Leites FS1
UFC FN 55 vs Rockhold Fight Pass Australia
UFC FN 48 vs Cung Le Fight Pass China
UFC 100 vs Hendo 1.3M
When on his own Michael draws close to 300k
The difference is that Mike is from the UK and is Primarily responsible for the UFC getting a broadcasting deal with BT Sport and we have no idea how much that is worth. He has also been used as the main event for Fight Pass cards to bolster subscriptions which is the future of the UFC with the PPV model becoming outdated. Mike has massive value to the UFC beyond PPV but he is also a great foil for GSP as the sh!t takling bad boy.
Beating Woodley is simply getting the WW belt back which he already had and never lost. If Woodley loses to someone else then GSP fights them. Wonderboy, Lawler, RDA, Ceronne, Kelvin etc do not pose or offer anything to GSP beyond a WW title fight. That opportunity is there whenever GSP wants it.
Beating Bisping offers a title in a second weight class, offers more money and is an opportunity that goes away if The Count loses or retires. Fighting any other MW (Whittaker, Luke, Yoel Jacare) that takes the belt offers Georges none of those things. He is less likely to win the MW title from any of those except maybe Whittaker. If GSP had to fight Woodley or Bisping and had to win or be executed with no other considerations such as money or titles do you think he picks Woodley or Bisping? I dont think he cares either way between the two in that scenario. In fact Woodley is a much easier strategy, clinch and wrestle, make him work and tire him out. Can't do that to Bisping.