From others' comments, it sounds like it is indeed common to get what you were seeking, and ProDive may just have a more risk-averse policy than some (many? most?) Coz ops. Maybe the ops that let anyone show up sight unseen for the 8 am dive as the first dive of their trip are mainly those with enough boats that they can routinely dedicate one to an easy site even in the morning, and ProDive just doesn't do that. Maybe ProDive rigidly adheres to the classic Coz routine of a more challenging dive--somewhat deep, possible strong current, swim-throughs, etc.--in the morning, and the easier dives later.
For what it's worth, requiring a checkout dive at a relatively easy site is not uncommon in many places in the world. Whether you know you are being auditioned or not, the divemasters are sizing you up on the first dive of your trip. I don't mind being subjected to this time-honored procedure at all, but I would not tolerate an op that wants to delay the start of my diving by half a day to do it.
Whether ProDive was actually offering a 10 am option or whether it might have turned out to be a bait-and-switch tactic, as
@awap suggested, we may never know.
You mention the Keys, and in my experience, most dive ops there are pretty careful about who they take out to the deeper wrecks. Depending on whether they know you, or how you answered questions like "cert level," "number of dives," "date of last dive," "last dive (or number of dives) over 60 ft," etc., they may suggest you do a shallower dive first or hire a private DM. I don't believe most Keys dive ops will take you out to the Spiegel Grove as your first dive with them based
only on your c-card; they always ask some or all of these questions. You were in the midst of this kind of questioning by ProDive, and you gave up.