So many of those teams just can't get in MLS. MLS is not established, they're very clear about making sure their growth is sustainable. Many of these outfits just don't have the media market or potential fanbase required to keep themselves financially afloat or be desirable enough to MLS.

Rivalry makes atmosphere, one of the ingredients to a strong league. It's not so simple as rivalry makes football, it's not like people only tune in during rivalry matches. Champions league audiences are bigger than those for derbies, because profile, fame, and media markets matter. Atmosphere is something the MLS has already worked hard at building. Even without the kind of established rivalries in other countries, it's done rather well on that particular front. As a watcher, I don't really care if a rivalry exists. I am Johnny America, and I am one of a vast number of people. I cannot expect that a rivalry I actually have stake in is happening, that would be ridiculous. I care if there's good football being played, a decent atmosphere generated by the fans translating into a more enjoyable experience live or on tv, decent commentary, and just a decent level of entertainment value.

MLS atmosphere is better than on the field quality. MLS atmosphere is better than the production values. We don't need rivalry, we need better product.

That's not to knock rivalry clashes, there's a reason people fly down to Buenos Aires for the Superrclasico despite that league carrying little weight internationally. But that's temporary interest. Sustained interest must be built outside of rivalries, so international viewers don't tune in only for the major RM/FCB and Manchester derbies. Rivalry revenue can't provide what is needed for that, especially when the rivalries are basically forced and created with comparatively tiny franchises in comparatively terrible media markets that people simply are not and never are going to be interested in.