My limited knowledge in engineering tells me:
Titanium is expensive, stronger and heavier than aluminium, lighter and weaker than high yield steel.
The most significant advantage of titanium is the good strength to weight ratio.
In case of submarines, weight is not really a big concern, as long as the weight is manageable (less than the displacement and bigger than displacement - maximum ballast mass).
The mentioned points exclude the use of titanium. Since the designers decided otherwise, and surely had some good reasons, I wonder where my misunderstanding is?