Alphabet Inc. has become an artificial-intelligence stock, for better or for worse. Though advertising still makes up the lion’s share of the company’s revenue, investors have been focusing more on the part of Alphabet’s business that seems the best proxy for AI momentum.

Cloud revenue in the quarter of $12 billion—a 30% increase from last year—just misses analysts’ estimates of $12.2 billion.

Alphabet reported an earnings beat and strong cloud growth, but the Google stock is spiraling Wednesday. Here's why.