It's not just Apple working on Swift on Linux. IBM for example has done a ton of work. A Facebook engineer has contributed a HUGE amount of work to bring it up to speed on Windows (without WSL or Cygwin). And Google will end up contributing on all fronts if they continue to push it in the ML space.
Apple Swift developers are actively working Linux support, but I suppose it'll forever remain a second-tier platform to iOS & macOS. Kind of like C# to Windows, but Swift doesn't have its Miguel de Icaza to create its Mono on Linux.
I'm sure the Swift team is well aware of this cultural bottleneck, hence the current efforts to support Linux, but it remains a cultural chicken-and-egg problem.