Uber's 6% stock price rise on Monday suggests investors like the company's $2.65 billion acquisition of Postmates. And it is, in theory, a way to shore up Uber Eats' restaurant food delivery in some U.S. cities and consolidate the market.

But investors should curb their enthusiasm. Postmates’ share of the U.S. market in recent years has fallen—it dropped to 7% from 12% just in the past year, according to Edison Trends. The combined Uber-Postmates will still trail DoorDash in market share nationally by a fair margin. Postmates filed IPO paperwork in February of last year but didn’t pull the trigger, suggesting there wasn’t much appetite for its shares.