Sure, the development cycle from the MySQL team is, in many ways, impressive. However, if the user's temperament is particularly gun-shy toward new technology, the longer-supported feature is the more certain bet. In this case, these three major features are still relatively recent additions. Even as of MySQL 5.0, ACID (Atomicity, Consistency, Isolation, Durability) consistency is not guaranteed in the case of a crash when some kinds of stored procedures or functions are used to modify the database. (See Binary Logging of Stored Routines and Triggers.)

Availability of Certification
Some IT shops love certification. While MySQL does have a certification training program, its training availability is not nearly as widespread as for, say, Oracle or MS-SQL Server. In broad terms, even if IT staff with MySQL are relatively easy to come by, certification or ongoing training remains less so, with fewer third-party training sources available. For larger IT shops, the enterprise experience that typically follows the commercial database systems is also desirable, while some people with MySQL experience may have less depth.

A related issue is the availability of qualified third-party support. While the availability of support services directly from the vendor mitigates the issue, to some degree, the same factors apply if strong local on-site support is required from a third party.

Corporate Considerations
Oracle, Sybase and Microsoft are all publicly traded companies. Whatever one might say about the strength of MySQL's backers, the fact that the company is not publicly traded means the financials are not required by law to be a matter of public record. At the risk of being accused of spreading FUD (Fear, Uncertainty and Doubt), the relative transparency of a publicly traded company (rightly or wrongly) provides certainty, stability and security to some IT managers and those to whom they report. The old saying about nobody ever being fired for buying IBM applies here (even if IBM recently decided to start selling MySQL); dealing with a large reputable corporate entity does tend to help some people sleep at night, be they investors, PHBs (Dilbert reference: Pointy-Haired Bosses) or experienced IT managers.

Perception of Scalability
I've titled this last reason carefully. There is a fairly consistent perception among many industry professionals that MySQL doesn't scale well. This matter is debated by some, though most debates tend to munge the difference between scaling up and scaling out (vertical vs. horizontal). MySQL discusses scaling out more than scaling up, but lists scalability as one of the top reasons for using MySQL.