What are the historical circumstances that led to the Emperors of Japan not having the ability to abdicate the throne?
Edit:
I originally thought the question was asking why is abdication unconstitutional, so my answer didn't really cover why the Diet is responsible for determining the line of succession, and the Emperor has no formal control.
According to the Imperial Japanese Constitution (大日本帝國憲法) promulgated in the Meiji years, the Imperial House Law, which governs imperial succession, was not amendable by the Japanese Diet (Parliament).
The Constitution read:
Article 1 The Empire of Japan shall be reigned over and governed by a line of Emperors unbroken for ages eternal.
Article 2 The Imperial Throne shall be succeeded to by Imperial male descendants, according to the provisions of the Imperial House Law.
And in the Imperial House Law:
Article LXII. When in the future it shall become necesarry to amend or make addition to the present law, the matter shall be decided by the Emperor, with the advice of the Imperial Family Council and with that of the Privy Council.
In the Imperial Japanese Constitution, the Emperor held substantial constitutional power, including the power to command the military and the power to amend the law on the imperial household.
After World War II, however, the Japanese Constitution was substantially revised by the occupying American military. The new constitution was in fact drafted by the Allied GHQ, after a draft by the Japanese government (including explanations here), which deviated very little from the Imperial Japanese Constitution, was disapproved. You can see the extreme similarity between the GHQ draft and the present Japanese Constitution.
The GHQ draft, and the current Japanese constitution, granted the power to legislate on the Imperial House Law to the Diet:
The Imperial Throne shall be dynastic and succeeded to in accordance with the Imperial House Law passed by the Diet.
Also, the Emperor is stripped away all political powers and roles. As the Constitution stipulates:
Article 3 The advice and approval of the Cabinet shall be required for all acts of the Emperor in matters of state, and the Cabinet shall be responsible therefore.
Article 4 The Emperor shall perform only such acts in matters of state as are provided for in the Constitution and he shall not have powers related to government.
Emperor Akihito now is in an awkward position. It is established that the Emperor should not interfere with politics. Suggesting an amendment of the Imperial House Law can be construed as interfering with politics, so he is even unable to directly state that he wants to retire and advocate for an amendment in that law.
That's why, I believe, he made his speech, and indirectly communicated that he wishes to retire. If you read the transcript of Akihito's speech (I hope this being used as an example is not a violation of the 20-year rule):
In the last few years I have started to reflect on my years as the emperor, and contemplate on my role and my duties as the emperor in the days to come...
While, being in the position of the emperor, I must refrain from making any specific comments on the existing imperial system, I would like to tell you what I, as an individual, have been thinking about...
I am worried that it may become difficult for me to carry out my duties as the symbol of the state with my whole being as I have done until now...
As I said in the beginning, under the Constitution, the emperor does not have powers related to government.
Original Answer:
Abdication is not unconstitutional, it's just not currently allowed by law, which can be amended by the Japanese Diet.
The Japanese Constitution states that:
The Imperial Throne shall be dynastic and succeeded to in accordance with the Imperial House Law passed by the Diet.
There's nothing in the text of the Constitution that prohibits abdication. But according to the Imperial House Law:
Article 2. The Imperial Throne shall be passed to the members of the Imperial Family according to the following order:
- The eldest son of the Emperor
...
Article 4. Upon the demise of the Emperor, the Imperial Heir shall immediately accede to the Throne.
Abdication is not mentioned in the law, and there's currently no legal procedure available for an abdication to take place.