Mostly the difference is due to the method of filtering, and which thing is moving.
In a typical mash tun, the filtering system is rather coarse in comparison to the voile mesh filter and so you can't crush as finely as you can in BIAB. Moreover the filtering system relies on the grain bed to filter and lauter the wort. Lastly the thing moving is the wort falling through the grain bed. If it moves too quickly, too slowly, if it's too thick or if it's too thing, bad things happen (stuck sparges)
In BIAB, the filtering system is very fine, and you can use anything crushed as fine as flour, although some claim ill effects I haven't seen any evidence of it from those who do use a fine crush merely those who don't do so claiming to avoid it due to ill effects which they never tested. That's a whole other thread though. This fine filtering system, and the fact the grain is the thing being removed, allows you do crush as fine as you want, and use any mash thickness you want. Just keep your PH in range and you're good to go.