Few things I'm reading from Japanese people (albeit, I won't say this is unbiased observation) often cite unrealistic expectations of their partner's income, look, and personality.

So, I've looked into the government research about it, at least for income part as it is being tangible. [1]

As for income, there have been some research that lifetime unmarried rate would collaborate with income. (Mostly for male) [2]

Their research further says that "In 2010, for male percentage of marriage for permanent employee is 27.7% while same for temp workers are 6.7%, resulting in about 20% of difference, while for female it is 28.2% for permanent workers, and 25.8% for temp workers, shows the much less difference. Therefore the increased rate of male temp workers is contributing to unmarriage rate or marriage at an older age. [3]

[1] http://www.mlit.go.jp/hakusyo/mlit/h24/hakusho/h25/index.htm....

[2] http://www.mlit.go.jp/hakusyo/mlit/h24/hakusho/h25/image/n10...

[3] http://www.mlit.go.jp/hakusyo/mlit/h24/hakusho/h25/html/n122...