It's a matter of random chance - sometimes I get the chest on the first cast, sometimes it takes 30.
It always seems to me to be governed by Murphy's Law of Fishing - the chance of a specific item is inversely proportional to the fisher's desire for it.
I never get serps or MIBs when I go fishing for them, I only get pearls and Big Fish when I'm not looking for them (when I'm fishing up SOS, not fishing for fish & serps), and you seem to get fishing deco, when you've got a lot of it, but always seem to get the chests in the first few casts when needed stuff for deco, or to stock your fishing vendor.
BTW, note that, in addition to the pillows, shells, furniture, and chest, you can ALSO turn in the footwear with the shipwreck tag, the skullcaps, and the BODY PARTS for points.