How was Katrina
not an environmental catastrophe -- the casual definition that I find repeatedly is "An environmental disaster or ecological disaster is defined as a catastrophic event regarding the natural environment that is due to human activity."
Given that criteria, Katrina was most definitely an environmental catastrophe.
Now, if you mean "When was the last time a president visited a catastrophe that involved chemical/biological/radiological factors?" -- well, that's a bit more rare; but I presume that it's done not as a matter of appearing placid by the society at large, but as a form of damage mitigation before all the facts are presumed known.
it's essentially a rite of passage for the president to hug people in some tornado ravaged state for photos. I presume the risks outweigh the positive P.R. in actual danger zones.
tl;dr : it's easier to declare a hurricane or flood site to be safe enough for presidential passage, so it represents a fairly easy to coordinate photo-op. Things are shakier when the facts about the danger present are not done rolling in.
p.s. I feel the need to clarify something: what I mean to remind by calling Katrina an environmental catastrophe was the significant role that society had in amplifying the damage of the event through both careless engineering, careless maintenance, and careless planning -- which was then exacerbated by careless response.