Well I heard that Cincinnati and Indianapolis was held on green clay at some point in their history. It would seem pointless to hold the US Open on green clay and then hold the lead up tournaments on hardcourts. That would be like Queens and Eastbourne held on clay and then Wimbledon on grass. Also, professionally, hardcourts have not been around that long, only since the 1970s as far as I'm aware.
After the USO switched to hardcourt, the majority of tournaments in the US in the summer were still played on clay for several years.
look at the schedule for 1983
http://www.atpworldtour.com/Scores/Archive-Event-Calendar.aspx?t=2&y=1983
you can see 6 US tournaments on that calendar leading up to the USO on hardcourt that were still on clay as late as 1983(USO switched to hardcourt in '78) And Indy was a pretty big event, it must seem strange to you, but the top players still played that green clay event for years even though the USO on hardcourt was just around the corner.
The tour wasn't very organized until 1990(when ATP started) it was sort of a free for all, no tournament was required to be a 'lead up' for a major, they just did their own thing(its not easy for a tournament to just switch surfaces after many years of the same one)
Lendl won 3 events(and they weren't minor events either) in 3 weeks on 3 surfaces in '85(carpet, clay, hardcourt)
And in Laver's time it was even more chaotic, I think fans here seem to assume there was some sort of grasscourt season leading to the USO on grass in '69(or assume most of the year was on grass or something since 3 majors were on grass - not the case at all) but it was really a mix of surfaces. think Laver was playing events on clay & carpet right before that USO in '69. it WAS sort of like having queens on clay then wimbledon on grass, players had no say & the powers that be really didn't care. open era was so new, it took a while to work out kinks, I'm sure players were just happy to be paid, & not worrying about not getting real 'lead up' tournaments to majors. I think all this shows how much harder it was to be at one's best during the majors in laver's time & makes his achievement all the more remarkable(when you see how much complaining about something as irrelevent as blue clay it seems inconceivable any player today would stand for the much more extreme changes laver had to deal with. they were also trying some pretty crazy stuff at some events, new scoring systems. and tiebreaks used to be sudden death, not what they are today)
there were also a few years where the Italian Open was played after the French Open.
I think too many fans just assume a lot about the tour prior to when they actually followed it instead of doing some research about it.