ANYONE WHO IS READING CHI NO WADACHI PLS HELP ME

*WARNING:SPOILERS*
Manga: Trail of blood/chi no wadachi
.
.
.
.
.
.
.
.
.
.
.Why are seichi and seiko being sentenced to prison without a psychotic evaluation? I get that the manga happens in the 90's but why? Japan didn't do that before? Like the 90's isn't that far back and Japan was very modern and up-to-date with everyting going around.
Is it cause he(seichi) accepted the fact that he had committed the murder knowingly ?is that why they decided to sentence him? But still they should take his mental health into consideration and with all seiko said in the interrogation, that she saw seichi's face when she pushed shigeru down the cliff.

Sorry for asking this here,nobody was replying in the topics,pls help cause google and YouTube were no help to me. I researched and researched but it was to no help