Like, huge stars of yesterday can`t make a dent in hot 100 or billboard 200, they`re dominanted by early 20`s and teens musicians. Mariah Carey last year flopped hard with her album, Madonna flopped with Rebel Heart an its singles. Older rock bands don`t even appear anymore, Rolling Stones new album also was a huge flop.

I think its because when the fanbase mature, they like to stick to the older songs and albums, and don`t care that much about the new releases from their artists, but they still pay huge money for concert tickets. The younger artists have young fanbases, wich consume everything their idols throw at the market and go on downloanding songs, streaming, and making those events to watch youtube clips to make their idols go to the top. 

Old people like old things and don`t care about the new things as they always stick with the thinking " Oh, my days were the golden days, now everything is shit", in 20 years we are going to say Nicki Minaj was a great musician and the singers of the time sucks, it may be the old cycle again.

http://www.billboard.com/articles/news/1552545/bon-jovi-debuts-at-no-1-on-billboard-200-david-bowie-at-no-2

http://www.billboard.com/articles/news/1567354/black-sabbath-earns-first-no-1-album-on-billboard-200-chart

http://www.billboard.com/articles/columns/chart-beat/6605803/james-taylor-first-no-1-album-billboard-200

http://www.billboard.com/articles/columns/chart-beat/6502454/empire-madonna-luke-bryan-top-three-billboard-200

"Madonna's 13th studio album Rebel Heart debuts at No. 2 on the Billboard 200 with 121,000 units, securing the Queen of Pop her 21st top 10 album."

They probably can't because only their older songs are iconic. No one wants the new ones, because the new ones probably don't follow the trends of songs these days.

People that buy albums are mostly younger people and old artists mainly appeal to old people.

Well what do you know? I didn't even know Bon Jovi had a new album.

I remember reading in an interview from Motley Crue's Nikki Sixx explaining why new songs from older acts aren't successful. He said radio stations are reluctlant to play them because they are considered a classic act and should be played on classic stations, and classic stations don't want to play their new songs because they aren't considered classic songs. It makes sense as to why their newer stuff isn't as successful since they aren't getting any type of support/promotion on the radio.

"The youth have shit taste"
How did Xscape do?

Also, older people don't have the necessity to buy an album immediately upon release, so the sales get staggered for months and years on.

I don't even know that an artist I like released a new album, sometimes I find out two years later. For example, I just recently found out that Skunk Anansie got back together 5 years ago (!) and that they have two new albums since.

This is mostly because older people don't follow music shows and charts (because our artists are not present there).

I think it's because the arts thrive on bringing something new to public domain; new sounds, new styles, new talent. The old tends to fade from memory. It's the nature of the beast.

The whole "new stuff sux" mentality is just the human reluctance to change. Change is easy when you're young and still figuring out who you are and what the world means to you, but I imagine it's hard as Hell once you've made up your mind and settle down in your comfort zone.

"The world changes, we do not, therein lies the irony that finally kills us." -- Anne Rice

i think its mostly a problem of elder pop artist, rock bands sell a lot even if they are realy old if the album is okayish.