Because you didn't type it in and there's no redirect set up.

If you want to use https on Konachan, just change the http in the location bar to https and it should stay on https as you move around the site. At the very least, I would recommend being on https when logging in.

*Edit
Though direct user links like this can still take you back to http. If you want, I can write a simple userscript to keep you on https. It would be practically the same as the one I use to stay on .com.

*Edit2

Really simple script.