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/
> proprietary OS
Say what? SmartOS is based on illumos, the only free, open source SVR4 UNIX out there.
Get the code: https://github.com/joyent/illumos-joyent
Build it! https://wiki.smartos.org/display/DOC/Building+SmartOS+on+Sma...
Modify it: https://wiki.smartos.org/display/DOC/SmartOS+Developers+Guid...
For the record, there is no "proprietary [operating system]" involved here. The Triton stack (upon which Manta is based) runs on top of SmartOS[1], a wholly open source operating system built around an illumos[2] core.
If you want to store objects in a distributed way, why not just use something like RethinkDB, which has proxying/sharding etc. built in? RethinkDB even has a way to execute code with 'js'.
Or for serverless processing, why not AWS lambda or the crop of clones?