There are a few reasons I can think of as to why it appears slats/flaps were selected late.
One could be that in the short moment of deciding where to go the Crew wanted to keep the A/C clean so to keep the best angle of glide until assured of clearing bridges ETC.
The other is simply the lack of time. Not sure about the A320 but on my type it takes a good 20 seconds to select/run the first stage of Slat/Flaps - there are 5 stages in total.
Thinking about the incident in general, those few seconds after dual engine failure, after the Capt. took control and turned south were the most important seconds of this short flight.
He had 2 choices - go for an airfield or go for the Hudson.
Landing on a runway from that height - even if you had enough glide performance, would have been incredibly difficult, (the ever changing glide angle with lowering of gear/flaps). Obviously if you get it right the rewards are an undamaged A/C and unhurt passengers. If you get it wrong though there is a high chance that none of the crew/passengers will survive
The Hudson presents a different option. It's relatively an easy target to land on but there is a chance of some of the passengers drowning after impact if the hull sustains sufficient damage. The fact that they all survived is testimony to the well executed touchdown, a strong A/C, and quick rescue. Plus of course some luck.
The overriding point though is that I have had the luxury of days to think about all this - he had seconds. A gambling man would have gone for broke and tried for an airfield - luckily for the passengers 'Sully', it appears, did not want to gamble with ALL their lives.
He made the right decision.
