Because it produces U233 as well as U232 Separating U-233 from U-232 by enrichment would be virtually impossible. Not only is there a very small mass difference between them, but the U-232 would be decaying rapidly and spraying the enrichment facility with hard gamma rays, killing everyone who wasn't behind a few feet of lead. Enrichment facilities aren't shielded, since natural uranium (U-235 and U-238) decays extraordinarily slowly over billions of years, and decays by emitting easily-shielded alpha particles. The enrichment facility would then be ruined for any future use with natural uranium because of U-232 contamination, and would have to be abandoned, at a loss of hundreds of millions of dollars.
