[SpecialistK]

There's a few theories:

* the legal disputes with AT&T meant that Linux was an earlier Free and Open Source Unix-compatible and gained momentum

* the leadership of the respective BSDs are more conservative or "picky" with their merges, requiring sufficient documentation and cross-architecture support

* the BSDs are 4+ different projects rather than one upstream Linux kernel, making contributions harder

* permissive licenses mean that a company using BSD code does not have to announce or share their changes to customers, making BSD adoption harder to notice (unless you read all of the fine print)