Coldplay were a lot smaller in Graduation era in 2007. They were starting to get pretty chart successful in X&Y era, but this was partly by gaming the charts and not a reflection of super wide popularity in the US. Not all that many people used iTunes yet in 2005, it was only introduced for PCs in the previous year, and the chart rules were changed very soon before the release of X&Y in June 2005, to make digital sales count for the first time.
Coldplay had high digital sales because the limited number of iTunes users at that time were concentrated among a smaller number of upper income people who tended to like Coldplay, so with digital sales included, their single "Speed of Sound" (which met with a tepid radio reception, compared to say, Clocks) broke some surprising records, and Coldplay started to get considered as a really large act, which I guess was inevitable from the moment Clocks won Record (or was it Song?) of the Year at the previous year's Grammys.
But in America they still weren't gigantic, they were only very popular by rock standards, not pop. They were slightly less commercially successful than Kanye himself at the time, so a Chris Martin collab wouldn't help a Ye song enough with the pop market, to be worth alienating some hip hop fans who didn't care for a crooning British dude they never heard of.
Viva la Vida era is when they got their #1, their Jay-Z feature, their Hype Williams video, their (reasonably) critically acclaimed album, and crossed over fully with hip hop audiences. Kanye was still touring Graduation/Glow in the Dark in summer 2008 when VLV started to pop off, but it was a hugely important transition in his own career where his style was changing, as the tour progressed, away from soulful samples, piano etc (all those elements that make Homecoming good) and toward electronic pop, autotune and the innovations of 808s.
He could probably have squeezed one last single out of the Graduation era, but Homecoming (unlike his more electronic Graduation hits like Stronger, Good Life, Flashing Lights) was more backward looking, antithetical to his evolving "pop art" style at that time (at least American Boy is explained by being a feature for Estelle, not his own song) and besides, Coldplay had so many of their own singles to promote in summer '08 that it might even have been against their label's wishes to release that as a distraction in the middle of VLV era.