Until you're published you don't really need to market your manuscript (you might work on marketing yourself as a writer, but there's no product yet). And if you suck at marketing, don't market. Not every writer does, and it's up to you to decide how much you want to do. If having a website is all you can handle, just do that. Put your energy into crafting a novel that kicks butt so the publisher feels it's a hot enough property for them to do the marketing.
I don't think good books will eventually be published, because "good" isn't all that's needed. You can have a well written, professional quality book that has the same plot as nineteen others on the shelf and there's just no more room for that idea. Or it has a similar plot to a blockbuster mega-bestseller. There's not a lot that can be done if that's the case, not matter how good the book might be. You can try to put a fresh spin on it, or you can write something new.
I do, however, think a great book will eventually sell if the author perseveres.
As for your second question, I suspect you're talking about query letters here, since you're saying "be published" and not "sell."
If so, the basic rule of thumb is: (If not, you can stop reading)
If you get no bites from your query, it's either the writing or the idea.
If it's the writing...
Keep working on it until you start getting bites. Get into crit groups, post on SYW, read sites like Miss Snark, Query Shark, Pub Rants and learn how to write a great query.
If it's the idea...
There's not much you can do. If the idea won't fly, you can either revise with a new angle, wait a few years to see if it's timing on that subject, or write something new.
If you're getting bites on the query, then it's working. Depending on how many bites you get can tell you if you need to tweak further. If you're only getting one or two out of a dozen, you might want to sharper the query some.
If you're getting rejected on the pages, then it's the writing.
The idea got your foot in the door, but the book isn't holding up for whatever reason. Again, find a crit group and find out why, then fix it.
If the full is getting rejected, then it's the book or the agent.
If it's the agent...
Sometimes a book can be publishable and someone just won't like it. You'll usually get "not for me, but you'll probably do well elsewhere" type comments if that's the case. Or a book is close, but not quite there yet and needs more work, they might mention what you can do to improve it.
If it's the book...
Sometimes a book has an interesting idea but the execution needs more work to fix. Or it starts well, but ends flat, or bogs down in the middle. Something happened to either stop them reading it or left them unsatisfied at the end. Again, it's back to crit groups to find out what isn't working and fix it. Or it might be a book that no one can figure out how to sell.