Mark Kolber wrote
As to why, it's pretty hokey, but there's some theory going around
that the CFI-I is easier, so, if you have to have an inspector for the
first ride, it might as well be the easier one.

The real reason is a little different. For all practical purposes,
you can't rent a complex airplane that isn't decades old. Any
determined fed can ground an airplane that old, and that's normal
practice in many FSDO's. If you go for an initial CFI ride in many
cases you get three inspectors - one ops and two maintenance. The ops
inspector starts your oral, and the maintenance inspectors start going
over the airplane. Oral ends when they ground it, and you get a pink
slip and, if you have the temerity to question their determination
(airplane not airworthy because the placard is curled up/TSO tag on
seatbelt unreadable/repair or alteration logged in logbook is major,
not minor, and requires Form 337) or the inspector just doesn't like
you, you get written up for flying an unairworthy airplane as well. I
know people who have had this happen, and there's at least one CFI on
this newsgroup who has his own story of something very similar.

The CFII ride need not be in a complex airplane - and new full-IFR
C-172's are available for rent all over. It's very difficult to
ground a new airplane. Thus I recommend that anyone doing an initial
CFI go over to a place that rents new C-172's and do the CFII first.

I did my initial CFI in a glider for the same reason - a new glider
was locally available for rent, and there was no way to flunk it
because it was new and completely unmodified - everything was just the
way it came from the factory.

Michael