The first set of AD&D modules ever released by TSR were based upon a huge plot involving Giants of various stripes. Along with TOH, G1-3 are prolly the most universally played "high level" adventures in D&D history.
Most certainly giants, including the Frost variety, garner Iconic status IMO. They are as integral to D&D in my mind as much as Mind Flayers, Orcs, Beholders, Dragons, etc.
The only reasoning I can see is perhaps the 4E MMx's are going to follow a "challenge level" theme. I.e. MM1 will be a certain amount of low to medium challenge monsters, and subsequent MMs will be geared towards tougher opponents..which is not a terribly bad idea, if the books were going to come out on a more frequent basis than the plan so far.