Probably because, while synthetic oil like Mobile One was around in
1986, not many people were using it so there was no need to warn against
it. By 2000, synthetics became very popular and for some reason, they
don't want you to use them.
The reason is probably because they have no control over what synthetic
you use, and they are all different.

Are there SAE or equivalent standards for synthetics as there are
regular oils?

With dino oil, they are all very
similar and the only main difference (other than things like sulfur
content) is the additives.

In other words, all good dino oils meet the various industry
standards.

So they can do testing with a high quality
dino oil and be reasonably assured that the results are reproducable
with other high quality dino oils. But with synthetics, the base
formulas and properties can be very different. Just look at the
evolution of Mobile One and you'll see that it's formula changed
drastically several times from when it was introduced in 1973; 1992,
1996, 1999, and 2002.

I wonder why the formula changed "drastically" so many times? Were
there problems?

Others have mentioned how well synthetics work in their cars; but it
seems to me that comparison is flawed. My automotive 350, at 60 mph
and 20 mpg, burns three whole gallons per hour. My twin boat 350s, at
similar rpms are each burning twelve gph. The flame front/peak
pressures/wall temp inside each cylinder is much different than any
car engine. It isn't automatic that what works at 3 gph will still do
a good job at 12 gph (at WOT it is 22 gph per engine.)

www.mobil1.com/why/synth/history.jsp

Steve