Real reasons Supercorp will be canon in season 6 as told by a film maker a thread:

Reason 1 : They needed to tear down the relationship and rebuild it. Many fans have come forward saying that after season 5 they can see romance now. Power wise Lena and Kara were not equal and now they are after season 5. Setting them up for a fair relationship.

As for William he was a last try at male partnership. And since it tanked they will have to follow Supercorp in order to achieve the iconic superman and lois status. They were testing to see if they could keep their options open and it did not work.

Reason 2: I work in film do you know how hard it is to focus on rainbow socks and then back to karas face? At a 2-4 F-stop it is really REALLY hard. The camera barely keeps anything in focus at that range. So it was on purpose that they focused on the "rainbow" socks.

Reason 3: They rewrote Kara being the one to pursue Lena in the beginning. They have decided in some form that Kara will play hero and Lena will play the damsel. They did this to follow the hero trope. And now they have a perfect set up for it.

No other character besides the main hero, the main villain and the main love interest ever takes over a whole season yet they gave Lena a whole season for her development. And if she is not the villain then she must be the love interest.

The 100th episode is all telling. In a episode like this you can do anything especially with Mxy but instead they wanted to tell a story about Lena and Kara being linked throughout realities. As a writer it the type of story you die to write and they wrote it about Kara and Lena.

In film everything is on purpose and there cannot be many accidents. Wardrobe is picked by the wardrobe department after reading the script and then sent to the director to be approved. So the red and blue you see all the time? Its on purpose.