In-universe, it's pretty clear that space travel has become a routine affair. Flights take off daily from Gattaca (and presumably other locations as well) and the passengers are only given the basic rudiments of zero-g space training as part of their general fitness training. Overall, they appear to be treated more like airline passengers than genuine astronauts.
Out of universe, the consideration was one of budget and look-and-feel. With only a limited amount of cash at his disposal, Andrew Niccol chose to adopt a minimalist "the past is the future" approach, choosing items of classic Americana (cars, buildings, suits) that emulated a sort of futuristic 1950s. This evidently included substituting spacesuits for business suits.
"It's very difficult to design the car of the future. Especially when
you don't have the money to do so. And we both mentioned a car at
almost the same time: the Studebaker Avanti. A strange thing happened
with Studebaker over the years: they had these very sort of rounded,
bubble-type shapes, very curvy, then they gave the design to Raymond
Loewy, and for just two years they had the Avanti, and then they
obviously get cold feet and immediately revert back to the old design.
But that's a car that was so ahead of its time."
Comparing the challenges of directing commercials, which he's done for
a number of years, and directing a feature, Niccol finds it is "just a
marathon, almost a survival test. Making a commercial is a quick hit,
but making a film is stamina. If I could recommend anything to anybody
it would be just to go through survival training immediately before
embarking on such a thing. Especially when you're attempting to create
the future, because every object... people come up and say 'Well,
Andrew, what is the pen of the future; what is the light bulb of the
future.' And when you don't have the money to create those things, I
just decided to drag a lot of the past and the present into the future
with me, and use classic designs."