Why would a hi-tech interstellar society have a social pyramid at all?
I see TL10+ as effectively a post-scarcity society where most citizens will be relatively well-educated, skilled and affluent and you'd have to work hard or not at all to belong to a genuine underclass.
Also remember it is a 1970s American game and Americans had and have a uniquely expansive definition of 'middle class' so it seems entirely appropriate that chargen produces diamond characters.
Indeed bearing this and the actual realities of 1970s America society in mind and particularly given current American developments I'd posit that the lower class (say Soc <5 which in MGT is where you get a negative SOC DM and are literally discriminated against) is on most worlds often associated with a minority that is for some reason downtrodden.
Because the baseline upon which the
Traveller game (or at least the OTU and similar settings) was based was the Golden Age Sci-Fi literature of the 1950s-1960s, which was NOT post-scarcity (as can clearly be seen from the assumptions in the OTU Setting). Many or most of those short stories, serials, and books did have Interstellar Empires with a nobility structure and/or Trade Economics (and associated Trading Empires and Politics) - (Asimov's
Foundation Trilogy, the individual worlds in E.C. Tubb's
Dumarest Saga, the
Terro-Human Future History series books by H. Beam Piper, the
Dominic Flandry series and
Nicholas van Rijn Trader to the Stars Series from Poul Anderson's
Technic History, to name a few). These were the primary inspirations for the game designers at GDW of
Traveller back in the day before
Star Wars hit the Big Screen and before computers were miniaturized into the microcomputer (and Marc Miller has said as much and mentioned these very sources).
The default assumption of
Traveller is most certainly that the setting is NOT post scarcity. The idea was a game where people still had to
do things in order to make it big, make a name, or in some cases even survive. Not just simply let the Sapient-AI and its associated infrastructure do it for them.
As a system
Traveller was designed to be setting independent and "bare-bones"; you of course can use the ruleset to develop any kind of setting you wish, but a post-scarcity egalitarian universe was not the fundamental presupposition in the backs of the minds of the game designers for the default setting when they made the game.