The frequency of constitutional amendments/changes varies greatly from country to country -- it appears from your reaction that in Japan, it's very rare and thus more similar to the USA than say Germany. As for reasons a left-wing party could have to change a constitution: (1) the incorporation of some human right, (2) the nullification of some outdated criminalisation, (3) changes in the election system. If no such thing is necessary and/or on the DPJ's agenda, that's good.

_{*Lunatic*, n.
One whose delusions are out of fashion.}