It doesnt. Since 1997 the show airs on sunday mornings at 7:30 am.

There's schools in Sunday in Japan ?

People get off from school and work on Sundays. That's why Toei has their kids shows on Sundays like Kamen Rider, Super Sentai, Precure, Dragon Ball Super, One Piece and Monster Hunter Stories: Ride On.

They could keep in Sundays and change the time slot ?

Why?

To see if increase the ratings.

Ratings are not a major issue. Merchandise sales keep the series alive. Toei considers ratings to be a after thought.

That is the commonly held belief in US toku fandom, but I've never seen any convincing proof that it's true. Given the realities of TV broadcasting, the situation has to be more nuanced than that. Toei may not care about ratings, but TV Asahi is the network that broadcasts Sentai, and they absolutely have an investment in how many viewers their shows attract. You can see Asahi's concern over ratings reflected in the increased roles their network producers are receiving in Rider and Sentai.

TV Asahi may have a profit sharing deal with Bandai, but no one in the US fandom knows for sure. But even if that's the case, Asahi's increased involvement in production is a pretty good indicator that they're concerned about ratings.

From what I've know is that merchandise sales are normally more important to Toei than ratings. Toei canceled Dragon Ball Kai (before they brought it back in 2014) presumably due to poor merchandising sales. Even Kyoryuger had stronger toy sales despite doing poorly in ratings as well and we still had new Super Sentai after 2013.

Toei actually just stopped the show because the last episode would have aired after the Tohoku earthquake.