Realtek is the low-cost provider with the typical PC user's feature "checklist" already coded. Other manufacturers (AKM, ADI, Cirrus/Crystal, TI) offer parts with better performance, but typically at a higher price, and the customer has to provide at least some of the feature package (more or less, depending on the vendor). This makes Realtek the "why not?" solution for most motherboard manufacturers.
This problem was caused by Windows, which was created by Microsoft Corporation.