Music occupies a bizarre nether region of commercially codified art. All of us think of and genuinely consider musicians--even middling ones--artists. Of course each type of art, whether it be writing and poetry, or painting and sculpture, or movies and theatre, has it's own modes of consumption.
Most of us do not own more than a handful of pieces of visual art (and more often than not, just reproductions). Most books, we only get through once--most movies too, for that matter. Plays are an occasion. Music though, we tend to own--or, in the streaming digital age, access--large quantities and we play much of it repeatedly. We somehow try to absorb music as a work of art whilst handling it as a commodity.
By contrast, our brief and infrequent encounters with great paintings or sculptures are fleeting and profound. Music by your favorite musician? You've steeped in those works. They're a part of your DNA. If that musician provides a new record of strikingly similar work, it's inherently going to feel, well… redundant. You'll probably enjoy it well enough, but it will take a conscious effort on your own part to get this new, ancillary material to matter to you.
Artistically, that seems unfair. Consider how long visual artists--from Monet to Rothko--mined highly specific niches. They'd plumbed the depths of it, with little expectation to re-innovate. One or two sea level changes are enough for one career, thank you. This plays into what we expect of musicians: we want them to audibly change and grow with each new work or release.
Additionally, like movies (and, to a lesser extent, books) we simultaneously expect music to be art that also entertains. Entertainment and artistic value are, at heart, different beasts. Of course, great art can entertain us--but it can also challenge, disturb or confront us.
Take the most obvious example: David Bowie. His career arc is at once exceptional, but also strikingly archetypical. He kept himself relevant for years beyond the usual pop commodities' shelf-life by way of radical reinventions: post-hippie psychedelic-folkster to glam-rock visionary to narco-soul crooner to outer-space pop revolutionary to new wave champion. He even pulled out of a creative nose-dive by rightly predicting a return of rockist power and then turned to embrace industrial and drum-n-bass dance culture. Lately though, he's settled into a career sunset of give-the-people-what they-want-ism: a sort of soft-focus pastiche of his own past, puréed into a stew that everyone can enjoy but nearly no one will remember.
Commercially it's a bit of catch-22. You have to innovate--but, as entertainment, most your fans aren't really looking to be pushed too hard. As far as the market is concerned, incremental, lower-case-c change is best--but even that is subject to a law of diminishing returns. New work may hew too close and be viewed as redundant. Alternately you could eventually end up too far afield, leaving your dwindling fan base pining for your 'classic sound'. Of course, appeasing them would then be a capitulation to a sort of irrelevance.
There's no universal answer to this problem. I have several (very OCD) devices I use to try and continuously re-engage. Sometimes I'll pointedly listen to an artist's entire discography, making a playlist by taking one, single song from every album. Going back to that mix will give me a span of their work-to-date leavened by time and progress instead of weighted by popularity. Maybe I'll choose an album I feel I've neglected and try to listen to it every day for week (at least).
It's too easy to let the way we consume music overpower the very thing we are listening to it for. Ultimately, we should periodically remind ourselves: no matter how commodified music becomes, it's value is artistic.