Why isn't the 3D marketplace more racially diverse?

Short answer:
Because most of the vendors are hobbyists - they have told us repeatedly over the years that they only make what they are personally interested in.

Detailed answer:
The vendor that has made the most black characters for the Gen4 figures is Reciecup - go visit her homepage here at 'Rosity.  We get many great black male & female figures from her - ever wonder wonder why?

If you want more than the same generic Caucasian 20 somethings, as Wolf 359 pointed out, you will have to either do the work yourself, or shop elsewhere - or better yet a combination of both.  Fortunately, there are tools available to us to address these issues - if one is willing to actually use them.

As an example, if I wanted to 'age' a V4, the V4 Ultra Aging Morphs over at RDNA appears to fit the bill.  Some of the legacy figures have a number of older characters available for them - SM's G2 series along with Miki 2 have a nice selection of both races & ages - the fact that they are normally sized and proportioned is an added bonus for what I do.

If you pick up a copy of Texture Transfer and Texture Converter 1 & 2, you can move those ethnic skins over to other figures, which is always a plus.

As far as the "It's hard to adjust ethnicity from a base figure" nonsense, I would suggest that people take a look at characters made for the SM figures.  Vendors such as Tempesta3d have changed Miki 4 from an Asian to a Caucasian and to an African.  The same can be said for Jessi 2, Sydney (Cyllan over at Content Paradise), as well as Olivia going from a black character to a Caucasian character (courtesy of LuluLee).  It can be done, IF the vendor is willing to put in the effort.  Alas, most vendors aren't willing to put in the effort, when instead they could be making their 37th Caucasian figure or their 68th V4 clubwear outfit.  

There are most certainly not "many" non-Caucasians in the various stores - I own almost every one that has been released since the P6 days.  Whether you are talking African, First Nations, Indians, Asians, etc - all combined are less than 10% of what is actually available.  For example, I did a V4 character search here at 'Rosity - of the 583 characters made for V4 that show up, only 17 were black (>3%) - not my definition of "many" and don't get me started on Indians or 1st Nations characters.  

Although in 'Rosity's defense, a large majority (60+) of black V4 figures I own (and are available for sale at 'Rosity right now) don't actually show up when I do a search on V4 characters - (Departments - 3D figure Essentials - Characters - V4) which makes the counting (and searching) a bit harder.  When I did my search, not a single figure (50 characters for V4/G4/A4) by Reciecup showed up, and only 2 out of 11 of Tempesda3D's black characters for V4 showed up.  

Which is a critical issue for 'Rosity in and of itself.  'Rosity should hire someone to fix their "search" function - because it certainly isn't actually finding what is available here.  If I was a vendor, I'd be a bit pissed about this.

Another thing is grasping that 'Rosity is not actually the Alpha & Omega of the Poserverse storefronts anymore - the interesting things are happening at other storefronts, whereas the vendors at 'Rosity are for the most part, still stuck in a Poser 7 timewarp.  

This issue may relate to why there are fewer people here than there used to be.  After all, if I am using Poser 8 or later, there are other storefronts that sell content that actually takes advantage of the features added to the last 3 versions of Poser.