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I don't get why they wouldn't make a lower assembly for 1 1/2"?
CAAD X is an OTP bike priced from £800, why would they make allowance for people to retrofit a £500 aftermarket fork? IS is easier for frame makers to build, since the bearings drop in rather than requiring a press fit which is the case with ZS or EC. Doubling the bore tolerance from ±0.05 to ±0.10 and eliminating the head tube facing operation is one of those things which production engineer look at when they have to nickel-and-dime mass produced bikes to hit certain retail price points.
Cannondale don't seem to be in love with huge lower headset bearings anyway, the Supersix and CAAD12 only have 1¼" lowers.
Not if the current lower is IS42. If you had a ZS44/30 lower for 1⅛" fork you could use an EC44/40 lower to fit a 1½" fork, but I don't think that's what the CAADX has.