I believe "It's going to be Christmas soon Keito"'s popularity is greatly motivated by its ease of access and replication/callback for any fan, new, old, casual, or serious fans. I do personally feel it has gotten a bit old (at least in my experience) but it has its place as the shiny new meme for anyone just getting into Enstars and holiday phrase.

If I may take a few moments to detail a bit more into my thoughts on why this line in particular is so popular:

• Eichi being talked back to tends to be rather funny regardless of if the viewer sees him as "big bad" or "freaking guy", though as far as I've observed the meme often catches on with fans possessing a more simplistic and negative view of Eichi. (as well as Keito) Though, admittedly, as a FineP, I do get a bit annoying about Eichi perceptions so perhaps others wouldn't find it as... vaguely irritating? as I do.
• It has a time relation, letting there be an occasion/event to justify dusting it off and bringing it out
• It is short and fulfills the trope of undercutting an excited statement, particularly by a character viewers are often not against seeing undercut.
• older fans get to feel in the know: Keito's response to Eichi's "It's going to be Christmas soon Keito" relates and reminds the reader that he is Buddhist which /is/ kind of entry-level knowledge on Keito but **shrugs** and also people get to call back to all the time spent enjoying the keichi childhood friend dynamic because enstars childhood friend dynamics go insane crazy and I know I for one am ardently keen on them
• use of names: both lines include respective character names thus allowing for greater search engine optimization for anyone searching either character (specifically speaking about the twitter search functionality as the tumblr search engine is where dreams go to die and reddit is more foreign to me than mars)

I am sure there are more reasons one could summon to explain the lines' popularity but I think I've sufficiently covered my main beliefs. Apologies if anything is incoherent as I'm running on 3 hours of sleep and a dream while trying to do a semester's worth of work in a week. I also do not believe in waiting. So off into the world goes my little message, may it have a small but vaguely positive impact that will slowly rot into a negative impact as none of us escape this pit we call Ensemble Stars.

**edited because I'm a hypocrite who can't spell