Because it seems to be the default choice. Harnesses are more expensive and most parrots won't touch them since how you put them on requires trust from both parties.
And chains are also more widely available and so on. In fact though there are places selling harnesses,no one really buys them.
As for why he is not in a cage,I don't really know. On a side note,there is a new chattering lory,a few months after the black capped left.
To be honest I'd rather put a chain than harness since I bought one before but I ended up being too short (I wanted to fly Amoy),tangling up and uncomfortable (since Amoy also plucks) and the material is horrible too.
That is the only brand that I found here.
Also most 'bird people' here don't really know them well enough.They mistake 'keeping time' as experience in the field.
They are connected but not the same.