I think a large part of it is supporting cross-platform developers who might be mainly Linux based internally, so someone writing apps in node primarily on Linux (for example) can more easily test against MS SQL Server (as well as postgres/mariadb/...) and add it to the "supported" list without that being quite as much hassle as it might otherwise be. SQL Server working on Linux potentially increases its catchment area for devs and projects noticeably. I see that as a much more likely use case than SQL Server on Linux in production.
Now that practically all the features are available in the standard edition (depending on your support and scale-up/scale-out requirements) it could end up used in production for licensing cost reasons particularly if the availability/DR/scaling features work well (where previously if you needed Enterprise edition the cost of windows was a drop in the ocean - Standard isn't exactly pocket money cheap but still...).
Another PoV could just be that these days interpolarity matters much more to MS than it used to, as their key product focus is moving further away from the OS & individual services and more onto the platform with Azure - they don't care what DB engine you run on which OS, as long as you do so on their platform - hence moves to support postgres & mysql in the database-as-a-service-upon-platform-as-a-service manner that they already offer for SQL Server. Of course they'd prefer that you used Windows and SQL Server but not wanting to (or depending on the project, not being able to) use one needn't get in the way of using the other, not should not using one (or either) get I the way of using Azure.
Now that practically all the features are available in the standard edition (depending on your support and scale-up/scale-out requirements) it could end up used in production for licensing cost reasons particularly if the availability/DR/scaling features work well (where previously if you needed Enterprise edition the cost of windows was a drop in the ocean - Standard isn't exactly pocket money cheap but still...).
Another PoV could just be that these days interpolarity matters much more to MS than it used to, as their key product focus is moving further away from the OS & individual services and more onto the platform with Azure - they don't care what DB engine you run on which OS, as long as you do so on their platform - hence moves to support postgres & mysql in the database-as-a-service-upon-platform-as-a-service manner that they already offer for SQL Server. Of course they'd prefer that you used Windows and SQL Server but not wanting to (or depending on the project, not being able to) use one needn't get in the way of using the other, not should not using one (or either) get I the way of using Azure.