Southern California has a dearth of available repeater pairs because of the massive amount of repeaters in use.
I might be oversimplifying, but isn't this the perfect argument for certain digital voice modes? DMR for example uses TDM to put two voice channels in a 12.5 KHz band.
The lower HF bands when they're open worldwide WRT sunspots etc. Above 14 Mhz its not an issue.