Because the sensitive info that is transmitted is your username/password, so why force it by default in case someone is browsing as visitor in http-only (in case for some reason https does not work for him).
Once logged in you would stay in https, of course (I should then force the cookie & session to be useable in https only).

I've an idea of the advantages of https connection, especially in security terms. But still doesn't make much sense to don't force https for everyone, including "casual visitors". Why for them https shouldn't work, unless someone is still using Windows 98 or exclusively relies on wifi hotspots?

In case there is a problem with the certificate on the site or its config

There are site alias at the imcdb.org and I have noticed the link is being correspond to which alias is being directed. For example, I have seen posted link using this:

/vehicle_197012-AM-General-M-35-G742.html for https://imcdb.org/vehicle_197012-AM-General-M-35-G742.html

Here are the list of alias I have known:

http://www.imcdb.org
http://imcdb.org
https://www.imcdb.org
https://imcdb.org

What can I use the alias redirect when I want to embedded the imcdb.org link for those whose uses the other alias link?

Indeed it works in both http and https, with or without the www.
When just pasting the link (without tags) the site normally replaces protocol (http or https) and the domain (with or without www) by the one you're currently using.
For embedded links (in the url tag) indeed that replacement is not done; maybe I should add it.
I'd say it is better to use the https version, but with www or without is up to you