Well it's a matter of time and resources. The voice actors can't just avoid work while waiting around for the DLC to begin development so that they can be available. Sometimes VA's go into their next job and end up having scheduling conflicts as a result.
There's also the development resources to consider too. They could have the budget, but not the time because all the involved companies and people can't just stop new work to make themselves available to Smash's DLC schedule. There's also a tangle of legal rights to consider. For example, it could be against the legal rights Nintendo agreed with to have Palutena, Pit, and anyone else from Kid Icarus to sarcastically riff on a DLC character.
Hell in Smash 4, none of the DLC characters had special move variations like the core fighters had. So there's precedent on DLC characters not being fully fleshed like the core cast which now numbers into the dozens in Ultimate.