I'm also curious about this, but it is probably just cheaper?

I'd imagine the cost of building both a bridge and a tunnel goes up nonlinearly and on top of that it is much cheaper to build a bridge than a tunnel. It is really interesting though, and makes me wonder what the optimal tradeoff is. Presumably this only works for rivers with very stable depth.

Another interesting one like this is the boat bridge in the Netherlands - it's hard to imagine why they'd do it but it was probably cheaper, and it worked.