Consider: If cross-region travel ever does get implemented, it will mean there will not (likely) be any further instances of, for example, only taking NA servers down for maintenance, as happened only a few weeks ago. The result of announcing only one region will experience downtime would result in an absolute flood of players to other regions in order to avoid any loss of playtime. None of the regional infrastructure is likely capable of handling that influx.

Wouldn't be surprised if there are more behind-the-scenes considerations like that going into the decision, more than "noooo we like our semi-closed JP community."