There are two gaps actually: generational and geographical.
The first gap is generational because the tailoring tradition has remained largely with the older generation as jobs and skills have migrated to "softer" more cognitive skills. Tailors are getting older and their replacements are virtually nowhere to be found.
The second issue - the importing of foreign skilled labor - is a logical byproduct of the generational gap. The US has long relied on importing tailors from Italy, Greece, Turkey and Southeast Asia. However, as these countries continue to develop economically and experience higher living standards, the incentive to live in the US has declined and the supply of foreign workers appears to have dwindled. In a recent Wall Street Journal article, John Daniel, a cut-make-trim factory based in Tennessee, has gone as far as actively recruiting Turkish tailors and assisting in their emigration to the US.
This seems to be the only real solution for higher end, tailored clothing manufacturers in the US. Tailoring is an artisanal process based on tacit knowledge and practitioner-based experience acquired over time. The best tailors today seem to have worked with the best tailors of a previous generation. And so on. Once this "path dependency" is broken, the industry itself may founder.
On a related note, I came across this interesting discussion thread on tailors in Southeast Asia. AskAndy member Matt discovered a Vietnamese tailor Chuong, a 30-something fellow who directly chalks on the fabric - a technique called "rock of eye" requiring great skill and confidence as a tailor.
This interesting find is suggestive. The key to overcoming the human resources tailoring gap in the US and other advanced economies may be found in identifying a fresh source of semi-skilled and motivated apprentices in Asia, Turkey and elsewhere. This shouldn't be confused with outsourcing, rather it is to ensure skilled tailoring is a domestically sustainable artisanal industry.