Looking at just the station usage figures Selby has double that of Goole
That may be more about Leeds commuter traffic being more significant than those working in Doncaster (and, even if as many people in Goole worked in Donny as people of Selby worked in Leeds, Doncaster is a much easier place to drive into and park relatively cheaply in, compared to Leeds), so this may be more about everyday commuters/shoppers doing relatively local journeys than any indication of London demands.
That said, Selby seems a bit "richer" than Goole from my limited visits, so that would indicate to me that there's probably more demand for Selby to have a London service. Howden serves much of the same car driving market that Goole does (both convenient for the M62 and A614).
I can't remember whether HT or GR moved first!
My memory (and, it's going back twenty years) is that HT started going via Selby from day one, so that they were offering something different to the established BR/ GNER service - but GNER amended their service to "fight back" - maybe the fact that HT were providing a "new" town with a London link helped in their bid to enter the market? (I appreciate that Selby used to have London services, but I don't think it did at privatisation (?))