Advising Chinese e-commerce giant Alibaba Group Holding Ltd. on its upcoming initial public offering, expected to be the world’s largest since Facebook Inc.’s $16 billion debut in 2012, is unquestionably one of the most coveted assignments of the coming year. It was a feather in the cap of Simpson Thacher & Bartlett when it landed the role of issuer’s counsel.

But the competition was less than one might think. That’s because Alibaba has let it be known that it won’t be hiring any firm for transactional work that also handles significant deals for Chinese competitors like Tencent Holdings Ltd., Baidu Inc. or JD.com. That ruled out from IPO consideration a number of top firms that might otherwise have been in the running, including Skadden, Arps, Slate, Meagher & Flom and Davis Polk & Wardwell. But Alibaba’s restriction also applies to smaller deals and smaller firms.