As someone who has followed minor league basketball over the years (yeah, I've got to get a life!), I thought I'd throw in my 2 cents.
First of all, the current "ABA" is a joke. It is a pitiful low level minor league that bears no relation to the original ABA that featured Dr. J, the Iceman, Maurice Lucas and Snapper. Go to their website....there are no stats, most games have no box scores and most teams receive no coverage in their local markets.
My understanding on why the NBA doesn't have a true minor league system where a guy like Travis could be playing is because the players union will never allow it. The NBA does now have the NBDL and for years before that had a deal with the CBA as developmenal leagues. Those options are there, but the deal with the union prevents sending players under contract down for seasoning like in hockey and basketball. I guess until recently the colleges were like the minor leagues that developed players for the NBA, but now with so many coming out before they are ready there is a need for a system like baseball or hockey.....but it probably won't happen anytime soon.
Last....minor league basketball is a very tough business....primarily because of competition everywhere from college and high school basketball. That is why minor league baseball and hockey is so much more widespread....there isn't the same level of competition from the college and high school versions. Minor league basketball never works in big cities like those that were mentioned above. The only place it has ever found some level of success is in select CBA cities...usually in the upper midwest....usually where there isn't direct competition from major college programs...i.e. Sioux Falls, Bismark, Fort Wayne, Yakima, Rockford, etc. The NBA has tried the same model in the southeast with the NBDL and is failing tremendously. Their attendance numbers are pitiful, even compared to the CBA. I'd be surprised if the NBDL is around in 2-3 years.
As for Portland in the ABA.....I'd be surprised if that league is around in 6 months.....and either way, Portland is a graveyard for minor league teams....arena football, roller hockey, ABL, WNBA, indoor soccer, etc.