He basically said it hadn't been around long enough for them to ensure its durability and longevity. His concerns were raised with the domes falling off, as they aren't really held in place through a fool proof mechanical bond.
Yes, they were more efficient than the Osram GD's and SSC P4's, but their durability was a big factor. That and, IIRC, purchasing large enough quantities at a time.
Using the XP-G now would require a driver with more and different drive levels... If levels were added to increase maximum output (ie: 200 and 240 replacing 140 and 170) then you would need a driver of even lower, efficient drive levels.
For example, lets say that the Osram GD needs an average of 1mA to produce .07 lumens, then replacing it with an XP-G would mean an output increase to say, .13 lumens at the same drive levels. Meaning to maintain the same outputs currently, and the visually even spacing difference between outputs, the driver would need to efficiently deliver a fraction of the current now needed. (These numbers are all purely theoretical)
This would possibly necessitate a new driver design... Very expensive, especially if he continued to offer the 120's, 140's and 170's, as well as the 100 high CRI's.