Purely a theory, but Coventry Rugby Club weren't happy with us moving to Solihull when it was first rumoured as they felt it was encroaching on their turf (even though Bedford and Ampthill and then Ealing, London Scottish and Richmond are all closer to each other than Solihull Moors FC is to Butts Park Arena). Suppose they kick up a fuss with the RFU leading to Solihull no longer being an option?
In that event Wasps would need to find a ground in the West Midlands that they can play at, sufficiently far enough away from Coventry. If Birmingham had been ruled out that doesn't leave a lot of alternative options.
Whilst I am not thrilled at the thought of Wasps moving to Sixways, even temporarily, if it's a choice of Sixways or no Wasps at all, it's not really a choice in my opinion