They're quite popular for distributed audio systems in general (of which sound masking is one type). "Constant voltage audio" comes in a few flavors and 70v is very common in the US, other parts of the world often use 100v. Background music systems in retail, voice paging systems, etc use constant voltage hardware because its much better technology for very long cable runs, daisy-chained speakers, and centrally located amplifiers.

The cost is fidelity. Full-range audio transformers aren't cheap, so these systems usually make some compromises because your announcements or smooth jazz over the pasta aisle don't need to be true hi-fi.

Its cool technology. Most of the speakers have variable power taps, so you can run a bunch of them in parallel on a single line and control the actual volume as-needed based on where the speaker is deployed by varying the transformer tap on each speaker.