hi, it's a matter of consistency.

It has been discussed here: https://github.com/bloom42/sane/issues/3

TL;DR: modern languages (Rust, Scala... except Go) use spaces. Allowing both will cause pain. And if in the future Tabs alongside spaces is really needed, It can be added in a non-breaking release, while allowing then disallowing tabs if they cause too much pain is non backward compatible.