Manta doesn't run on a proprietary OS, but runs on SmartOS, a distribution of illumos [1][2]. In fact, almost everything at Joyent is open source, including all of Manta [3]. As for why not anything else, Manta has been around since 2013 [4], before other similar solutions, and allows you to run arbitrary programs on your data. As an example, some of our customers run ffmpeg on videos that they upload, to produce different variants to then store in Manta.

[1] https://github.com/joyent/smartos-live

[2] https://github.com/joyent/illumos-joyent

[3] https://github.com/joyent/manta

[4] http://www.theregister.co.uk/2013/06/25/joyent_object_store/