I believe part of the reason was also some experience with C++, in which you sometimes have to "unconst" some fields of your const classes (the mutable keyword). This is a really ugly and nonintuitive design, so I assume they'd rather take extra care to make sure they don't have to repeat it. Even if it means no const at all.
I don't think this kind of thing is all that non-intuitive if you reframe const as shared vs. unique references. Rust is a good example of this, although with Go you would want to sidestep all the Cell stuff since it's unnecessary.
