Because is an amazing movie, a real classic.

Because of the starting scene of BvS, and BvS story is influenced by Excalibur

Indeedthe cast was literally packed with great actors, many of whom went on to be superstars in their own right - Liam Neeson, Gabriel Byrne, Patrick Stewart, and as you say Williamson and Mirren.
Whose idea was it for the word "Lisp" to have an "S" in it?

You don't need to be into the Arthurian legend to enjoy this film. It has so many great performances, a classic love triangle and Nicol Williamson and Helen Mirren.

This movie is a classic. It was my first exposure to the Arthurian legend. Also really made an impact on me being so young. Seeing it as an adult, it really is a good film with an awesome soundtrack and really solid acting. Terry Nigel is amazing. Watch it.

Is this a recent developmentas in the last week or so? If so, as pol-edra said, it's likely due to Nigel Terry's death.
If it's recent
ish
say the last six months or soit could be due in part to the fact that Guy Richie is currently filming the first part of a proposed King Arthur saga. That's bound to have sent younger people looking for previous films on the legend.

A journey into the realm of the obscure:
http://saturdayshowcase.blogspot.com/

Then the visuals in this movie should agree with you. it's mostly lights and mists.
"
Occasionally
I'm callous and strange."

I actually tune out when scenes are fully CGI so I don't mind low-tech special effects at all.

I honestly have no idea. All I can tell you from experience is that for many of my students (currently in their twenties), this movie "looks" too old for them to enjoy it. Then again, some young viewers don't mind that, and actually enjoy a change from CGI-heavy modern movies. I guess it depends on your sensitivity to cinematography and not just storytelling.
As for the genre: well obviously if you get bored at the mere mention of knights, quests, or holy grails, then this has close to zero chance of appealing to you. If on the other hand you have some interest in fantasy in general or in the Arthurian legend in particular (which I assume you do, otherwise you wouldn't ask about it), then I would say it's a must-see. Just don't expect LOTR, it doesn't look at all like it, it's something else.
"
Occasionally
I'm callous and strange."

Cool. Do I need to be into this genre to enjoy it?

It might have something to do with Nigel Terry just passing away, and this being probably his most famous part, or with the upcoming Arthurian movie directed by Guy Ritchie, which is prompting renewed interest in this classic (some might say definitive) version.
"
Occasionally
I'm callous and strange."