Well, I don't know how Steve coded things, so it is entirely possible that 'having had a colony on the body at any point, for any reason' is enough to remove them from the list of CMC sites. Otherwise, it may simply be a case that the Sol sites are sufficiently numerous and/or high quality, that 99% (or more) of the 'growth' goes there instead of the other systems. I do recall there being weighting system to the random allocation that makes not all sites equal.
Finally, as a last guess, I have a vague memory of a discussion of upping the requirement from 10 million population to 25 million (to be the same as the source/destination for colonists cut-off) or 40 million (for reasons that I definitely don't remember). I don't think the CMC code was actually changed, but only Steve could say for sure.
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Edited to add: Ooh, one last thing. It may be the requirement is "10 million pop and a spaceport" rather than "10 million pop" alone.