it is too bad.  i think part of it is pressure to produce, but the other part of it is--laziness?  i don't know.  but i read somewhere---might have been in the writer--go figure--that james patterson doesn't even write his own books.  what they hey?  i've never read patterson, but he does a story outline and others write it, and then he reads it and adds changes.  that isn't being a writer...bogus.  i just finished twilight--which was okay.  all my students, or the girls, are bustling aobut it. it wasn't great.  the story line is the popular thing---and i've read plenty of books that i wouldn't say are wonderfully written, but i really enjoy the story.  not even that much with twilight.  i guess i am ranting---the future classics?  hmmm, if my students are any inclination---no one will read the classics.  lol. um, i think the classics will come from less prolific but stunning authors like ian mcewan--who will never gain superstar status, but whose books are an experience.

 There are various reasons. They might include:

(1) An author starts out full of energy and ideas,and they work just fine,and he/she builds a name. The author has a lot to say.After some years, he has run out of the things he really wanted to say,without repeating himself again and again,and he tries for books equal to early successful ones,but unless a new idea suddenly strikes,he is going downhill. You can't be world champion forever.

(2) He makes a splash, publishes his best stuff,and his publisher wants more....his agent/editor asks if maybe he has something from his early days that was rejected originally,and off they go  again. But YOU notice the difference.

(3) Often,though not exactly the "over the years" way you are meaning, an author will not get his first book published,but numbers two and three really take off....and then the publisher keenly publishes the first but unpublished book,which may well have been from the writer's "learning period",or an okay book but a totally different genre,which does not suit the tastes of his fans for his newer books.

(4) The publishers asks for more books.The author previously did one book in about three years or more,and now he's expected to  do at least one book a year and still keep the same quality level.That is probably difficult for  even a topnotch author,though there are exceptions.

(5) Same problem as number (4),but the author solves it by getting other people to write under his name.They write the book,meaning it's very different to what you expect,but his name is on the cover and the general outline is the same. Sometimes an author will team up and have both names on the cover.

(6) Sometimes the writer has gotten rich and successful and,in a sense,couldn't be bothered.He's now found so many other things to give him a full and happy lifestyle.

(7) Sadly,sometimes the author has  turned into a drunk or whatever.

(8) Sometimes it's just old age that comes to everyone. He still might be  very intelligent,but if,say,he was a thriller writer,can no longer  feel the empathy for his hero  going into highly dangerous sitiations  that he may have found exciting when he was a younger author.

(9) And maybe he has been asked by his publisher tio make the books more politically correct,cut down on the violence,cut out whatever,add more swearing and slang and sex or whatever.....and you,the loyal reader,finds the new books not so good,and the author would probably agree with you.

And there are  probably many other reasons.