This is pure speculation, of course, but I almost wonder if they decided that adding Spidey in to CA:CW wouldn't have worked from a story perspective (rather than a financial one. I mean, there are only been 3 ways to incorporate him, and none really were options for various reasons:
- Merge TASM 1&2 with the MCU as cannon and with Garfield returning as Spidey. This was apparently least palatable to Marvel because the leaks suggested he would not return (which is a shame, because though I didn't like the movies, I liked Garfield as Pete/Spidey). Wise choice as neither TASM or Avengers shows signs of each other's presence, and people would wonder where was Spidey during the "Battle of New York"
- Treat CW as Spidey's origin story, which would then lead into his appearance in IW 1&2 (and likely solo movies set in the MCU). This wouldn't work as CW is going to have too much going on to have room for an origin story, plus Spidey's role in the comic story would require an established, well-known superhero in the movie. BP serves that role (only in reverse) better than an origin Spidey.
- Introduce Spidey as an established hero in the MCU (ala Batman in BvSoH) without showing his origin. I suppose this could work as most people know his origin story already, you just run into the same problem as fusing TASM w/ the MCU: where was he during Avengers 1&2, IM3, CA:TWS and AoS?
I suppose they've going to run into the same problems if the deal gets worked out for IW 1&2, they just have more time to craft a narrative that fits in with what they've created already. Who knows, a new Spidey solo origin film setting up his appearance in IW could still be made, it just makes my head spin we could be seeing a third Spidey origin in 15 years due to Sony's malfeasance.