I think one reason the old basic sets covered levels 1-3 was that changes to the rolls to hit or save did not kick in until 4th. The few more column inches was probably not so significant (See the combat matrix for monsters attacking.) as the neat simplicity in presentation and practice.
Spells were the big deal, but another 16 pages or so (e.g., taking the Holmes Blue Book up to Moldvay's 64, or Moldvay's presentation up to 80 pp.) could have been plenty to cover that. (Magic items, monsters especially relative to Moldvay, and of course wilderness adventures and the power-politics "endgame", could still have yielded a second volume.) Editorial concerns meshed with commercial ones -- but in a way that to my mind delivered excellent value for players, versus the "white box" plus supplements that the "B/X" sets replaced.
Powers take up many more pages in the 4E PHB. That's a basic element of the game design, not just the presentation. There's a built-in trade off in which more levels = fewer classes, or vice versa.
1 extra level certainly would mean new 3rd level powers. That might be a reason why they decided for 2 instead of 3 levels. I guess that would mean about 4-8 extra pages (for 2-4 powers per level.)
It might also contain magical items, and a 3rd level might warrant new magical items. If they keep the treasure parcels around, this would mean up to 8th level items, I think. Probably another 2-4 pages at least (spread over weapons, implements, neck slot items and armor).
And of course, if their goal is to include adventure material for 1-5 players for 3 levels, this would also add extra pages. Probably about 20 pages (assuming 10 encounters á 2 pages and no additional non-combat texts - which of course is unlikely, but then, some encounters might be harder so 8 encounters might be enough.)
So the cost of one additional level and getting it "right" could be about 32 extra pages. I guess that would be another 10 $, judging from the Dragonborn softcover book (IIRC).
Aside from Paragon Paths and Epic Destinies, at 2nd level the players would have "seen" all the mechanical aspects of a 4E character.
Ability Scores, Background (if they include them?) Race, Class (with Build Option), Feat, Skills, Encounter Attack Powers, Daily Attack Powers, Utility Powers. To learn how the game plays, you should have everything at 2nd level.