I think the biggest motivation for excluding women as successors is to limit the number of potential heirs and to concentrate power for the reigning sovereign. Furthermore the reasons against doing so are weak.
First, a background of Japanese empresses. From Wikipedia:
- Empress Suiko (554–628), r. 593–628—first ruling empress
- Empress Kōgyoku (594–661), r. 642–645—formerly Princess Takara (Empress Consort of Jomei)
- Empress Saimei (594–661), r. 655–661 (same person as Empress Kōgyoku)
- Empress Jitō (645–702), r. 690–697
- Empress Gemmei (661–721), r. 707–715
- Empress Genshō (680–748), r. 715–724—formerly Princess Hidaka
- Empress Kōken (718–770), r. 749–758
- Empress Shōtoku (718–770), r. 764–770 (same person as Empress Kōken)
- Empress Meishō (1624–1696), r. 1629–1643
- Empress Go-Sakuramachi (1740–1813), r. 1762–1771—most recent ruling empress
It is noted that 8 of the 10 listed are between 593-770, relatively early in Japanese history. Furthermore, they were all princesses (having fathers who were emperors) and their successors were all chosen from the paternal imperial lines. That is, no empress inherited the throne via the maternal line.
Given this, it can be argued that empresses are unusual, and do not enjoy the same "legitimacy" as emperors. In a few cases women ascended the throne to postpone succession conflicts between male heirs. Therefore the reason for including women in the succession rules is not so strong.
Japan did not practice primogeniture for a long time. What used to be the norm was that the title would rotate between brothers in order of age, before being passed to the first son of the eldest brother. The emperors also needed to perform many time-consuming rituals, and abdication (rather than death) was more common, after which the emperors enjoyed retirement in luxury.
The relatively short reigns meant that there was more of a need for succession candidates, which meant less of a reason to exclude women in the past.
Changes under Meiji Restoration
The Meiji Restoration was a period of rapid reform and modernisation. The Meiji Constitution gave the emperor a lot of power and established a constitutional monarchy heavily influenced by the Prusso-German model.
This background is important in that it hints at two factors that contribute to the exclusion of empresses:
- Excluding potential successors and concentrating power on the monarch. The new emperor has a bigger and more direct political role and much less of a ritualistic one, which meant that their reign could be and needed to be longer - rapid succession of such a powerful figure would be highly destabilising. This also means that there is less of a need for more successors, and too many potential successors is both a destabilising and power-diluting factor.
- Imitation of the more advanced Western model; this reform era was so rapid that there was no time to thoroughly prove every small reform. "What works for the Europeans will work for us", was what Japan probably felt. Although some exclusions were made, as the question mentions, the question of empresses is a relatively small one, since the tradition of empresses is quite weak.
There have been calls to loosen the succession rules, some by removing the exclusion of women, in fairly recent times. However there are many motivations for this, and the traditionalist one is fairly minor.
- The chief concern is that some consider the pool of successors to be too limited. The 1947 Constitution of Japan further limited the succession rules by excluding adoption, polygyny and non-direct male descendants. The motivation at the time may have been to severely curtail the influence of the imperial family, but this also means that if an emperor has no sons with his only wife, then the imperial line could die out. The current line of succession is that the crown prince is more than 50 years old and has no male heirs of his own; furthermore his brother had no son until 2006, prior to which there was increased talk of loosening the succession rules.
- There is also some vested interest in restoring empresses by the current princesses.
- Another likely motivation is of an egalitarian nature.