Sorry for replying an old post. Though published manga is shrinking, the growth of digital manga seems robust.
http://i1373.photobucket.com/albums/ag394/poplarbrook/Secret /9F5CFE7C346BEE5D5FA5B2CDF09F81F5_zpsmua4uzkg.jpg
The orange trend line is the sales revenue of digital manga, calculated from digital books multiply by 80%.
http://ajpea.or.jp/statistics/img/2014_comic.jpg
The blue line stands for manga books and green one means manga magazines.
I assume that digital manga serve as alternatives of manga books, resulting in shrinking trend of published manga.

Since 2000 decline trend has been observed of manga published in Korea and Spain. However in France manga even outsale BDs in 2013.

Other source I have read mentioned that manga occupied 60% of the published books. There is even one piece of news said light novels take up 50% or something of retailed book market - however I discovered that retailed book market was mistranslated from bunkobon market. Light novel market is really dwarfed by manga market

I find other sources say that graphic novel market in 1993 was much larger that it was in 2000 (though the market size has restored years ago). I am not sure, but it may be applied to manga market. Many manga series nowadays cannot match their counterpart in 1990s in turns of sales per volume published.

As for the main topic, I think art style matters just as other replies do. Manga art style is more inconsistent through time than other comics are, I think

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So this is where the pm questions came from.
US comics utterly collapsed in the 90 and Marvel even went bankrupt. Watch this 13 part web documentary for the cliff notes version (just browse his channel for the rest).
This amazing documentary further talks about all sorts of problems while reaching from the 30s to the 00s. The US comics market was on the road to "recovery" (the glory days are forever gone though) and 2015 was the best year in ages but it ended with a BIG sales collapse for most and all sorts of problem. Just research on http://bleedingcool.com

Manga also currently sell better than homegrown comic in the US (the US manga market is way smaller than the on in Germany or France btw.) and other places but the whole thing is way more compacted that one would assume. You all will have to do your own research as i would need a few thousand words to even end up somewhere. Let´s just say that i read a few dozen books on the history on international comics and move on...
Adaptations though are all the rage so comics are still needed! They must lead and not follow and never forget that most modern anime are also adaptions. The Japanese manga market is also in a transitional period with publications closing left and right but this topic is also endless and a bit controversial. The big sellers make money like never before but the bottom is falling out is my one sentence summary of the dilemma.

The lack of interest for "old" manga ( and modern content that further appears to be dated) can be proven with this very database. Just look @ all the over-the-top Seinen series from the late 70s to the early 90s by KOIKE Kazuo and co.
They are even more over the top then their current brethren, the emerging low budget / trashy Korean web-pr0n comes to mind , while having amazing production values but almost none are in translation despite the palpable interest for such content on viewers. Why? Because all the new macho trash is already in scanlation and there is only so much time in the day on all sides. They are also your dad´s wanking material if the scene had existed back then. Insert These "This Ain't Your Daddy's" jokes [here] .
I continuously redo and set up all sorts of classic (or at least "old") MU/Wiki entries to give such series at least a chance @ life. Don´t complain that someone should do something if you are doing nothing i always say! The journey of a thousand miles begins with one step.

The Killing Joke is also nearly 3 decades old but was still in the top 10 bestselling Graphic Novels of 2015 (the toon adaptation should be the main reason here) and i mentioned recent manga revivals in my previous comment. If you build it, [they] will come is true and actual literature tends to stand the test of time so don´t despair just yet.
Most of your jobs shouldn´t be on the line if people stopped reading... Hm.

Last edited by residentgrigo at 5:46 am, Feb 28 2016

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I also read EU/US comics and am a librarian.
Manga-Masters, My ANN-Lists + Imdb

For one, it's the art style.
Another reason might be they are just not what's trending anymore, and that older manga = people already read it before.