The South Loop as a residential neighborhood was virtually non-existent 20 years ago! The boom south of Roosevelt Road started as interest rates on mortgages decreased and buyers began craving new construction. There were so many developable parcels that it was a developer's delight. Dearborn Park and Printer's Row have been around lots longer; they are farther north - between Congress Parkway (which is the southern border of the Loop financial district) and Roosevelt Road.
The housing stock is nearly exclusively "luxury" condos and townhouses in developments. Up until recently, one of its claims to fame was that Mayor Daley used to live there. The Mayor could bicycle or walk to City Hall from there, as can you if you live in the South Loop. Being just a few minutes from the Loop is appealing to time-starved professionals. Plus all the big and wonderful lakefront museums are right there, in the relatively new and gorgeous Museum Campus. Monroe Harbor is nearby so it's a good location for boaters who moor there, and of course, the lakefront park is right there too.