Haven't the Grand Prix fields already been expanded to what they were - they went back 12 singles, 10 dance, and 8 pairs.
And if the top junior dance teams cannot earn a guaranteed berth, they cannot be sure of getting an invitation on the GP no matter how many spots are available. Every GP host country has something at stake by inviting a top junior dance team to their GP event. So much easier to skip the top junior teams and invite a lower-ranked junior dance team from another country--one that hasn't been earning perfect levels this season & isn't likely to muck up one of your own host country dance teams' opportunity to earn GP points.
At the moment, the only way a junior dance team can earn a guaranteed GP spot is via world standings. And that maybe means putting in three years at the highest level on the JGP. Should a Junior World gold medal dance team really have to do that?
Of course not.