To directly answer your question, it is a guess, but I'd say that politics prevents it from happening in the future, and momentum prevented it in the past.
As to politics, it's hard enough to get a bunch of grandmasters to agree on what to have for dinner, let alone agree to working together on inter-organization support for a global effort - every 4 years.
As to momentum, I think that each of the sports you mentioned (tkd, wrestling, judo, etc) got there on their own merit at different times. At the time, no one thought about combining them. Now that Taekwondo and Karate are about to show at the same time, such a question is easy to ask. Sumo also tried and failed to get in; that would be a parallel comparison to wrestling, and the same question arises: why not have one category, then lump all martial arts (tkd, karate, etc, plus the ones trying to get in) underneath that umbrella, either as a sport as they currently are, or as events, or as disciplines.
I also don't know if creating "martial arts", and then lump tkd, karate, judo, fencing, boxing, wrestling under it would shrink 28 down to 23, and then we can add more events under martial arts with impunity (eg, wushu and sumo), and then add more major sports as well to stay under that 28 limit.
Because aside from politics and momentum, there's another reason: money.
The reason 28 is set by IOC is to prevent runaway budgets a country has to incur to host the games. Whether wushu is a sport or an event, the cost to cover the entire games rises, because media time, commentators, venus, security, etc all have to handle the additional load. As to venue, perhaps the cost differential might not be that great - they can re-use it for similar sports - but still, the fact that people are there means additional infrastructure, eg security, still must be paid for.
I would love to see a great many more styles represented in the Olympics. The popularity of one style - like Taekwondo - reverberates to other styles as well, so as a community, everyone benefits.
Of course, many of us in Taekwondo also bemoan the damage the Olympics has done to the style. This is an area that doesn't answer your question about why can't there be a better structure to allow for more styles, except to say that not everyone wants more styles - that's part of the politics part.