Oh, you say because Wikipedia is full of errors, whereas those guys at Britannica are paid to get everything right. You're going to be disappointed then to learn that there are lots of errors in Britannica and moreover once they are identified fixing them is hard - impossible if you own a print version. Wikipedia has an entry called, “Errors in the Encyclopædia Britannica that have been corrected in Wikipedia” that lists some notable “mistakes and omissions.” The Wikipedia entry says:
“These examples can serve as useful reminders of the fact that no encyclopedia can ever expect to be perfectly error-free (which is sometimes forgotten, especially when Wikipedia is compared to traditional encyclopedias), and as an illustration of the advantages of an editorial process where anybody can correct an error at any time.”
Wikipedia and its rise over Britannica features in Chapter 11 "Web 2.0" of The Universal Machine.