SIG 1911s are relatively new to the market so maybe they just don't have the appeal of the Colt name for dropping $2000-$5000 into for a full custom. I bought a Sig Spartan Carry quite frankly for the Bronze finish and the Spartan grips. What can I say, just looks cool to me. So I bought it with the intention to leave it stock for the most part because of the finish. I haven't shot it a ton but it is more accurate than I expected and has been 100% reliable. I know early on I heard extractor breakage issues but I didn't follow those stories at the time because the Sig 1911 line didn't really excite me. At any rate, EGW makes an improved extractor so I may do that to it but otherwise it will be the one of a few stock 1911s.
Truth be told, Colt and maybe Springfield would be the only two production 1911s I would invest in customizing. I'm good with some of the aftermarket sets like Caspian, STi, SVI, etc. but for a factory gun Colt & SA would be my limits for factory, production grade guns. There is just more nostalgia with them IMO. That is not to say that a SIG 1911 wouldn't make a good custom base. I think their quality is good, and in all honesty if you are getting a full custom the frame & slide are likely the only original parts left.