There are several reasons for this. Originally, the biggest was legal: When OSSv4 first came out (2002), it was not free software, so it could not be included

There are several reasons for this. Originally, the biggest was legal: When OSSv4 first came out (2002), it was not free software, so it could not be included in Linux. In 2007, it was released under the GPL, so that went away. However, this created animosity towards it from the kernel devs. That explains OSSv4, but what about v3? OSSv3 was the only sound system in Linux 2.4 and earlier; why didn't they continue developing that? There are various criticisms of it; that it doesn't map well to modern audio devices. I am under the impression that a lot of the design criticisms of OSSv4 are actually against OSSv3, and they haven't bothered to learn OSSv4; but I have no real citation for this. The OSS people don't really work with the Linux people, so the Linux devs don't want to invest a lot of time in someone else' code. This makes even more sense when we consider that Lennart Poettering claims there are only about 3.5 paid people working on sound in Linux--they would of course be fairly heavily invested in ALSA.