Originally Posted by Urlik
I think a lot of the writer then wrote short stories because most of them got published in magazines and genre collections.
That was indeed a part of it; and with the early (published, not series-chronological) Elric tales, it was certainly a factor. But if you'll look at the majority of books in general, and especially genre publishing ... there simply weren't any of the huge tomes we see today -- at least, not unless they were anthologies gathered from many sources, as with some of Groff Conklin's, or Anthony Boucher's Treasury of Great Science Fiction (2 vols. -- just over 1000 pages), etc.
For one thing, there wasn't much of an audience for long, drawn-out stories at the time. Most genres were considered just relaxation reading, and not to be looked at as literature; so they had to be more brisk in pace to hold the reader's interest. Also, a lot of writers wrote both short stories and novels, and knew how to keep a story to its essentials, instead of the often bloated works we see today (and have seen before in other fields -- the Gothics of the early 19th century come to mind here). "Less is more" was still a good way to work... it whetted the appetite without satiating -- or surfeiting -- it.
They didn't get big advances, so they had to turn out lots of material to put bread on the table. Also, Moorcock has always worked incredibly fast (with rare exceptions such as the Pyat books), sometimes turning out several novels within a handful of months. (The Michael Kane books, as I recall Justin Leiber remarking, he turned out in one week -- all three in one week, that is.)
Sf, especially, was still predominately an adventure-story genre; the meatier, thought-pieces, such as Earth Abides
, being the exception, rather than the rule. That sort of book can seldom really be maintained over a long stretch without becoming very attenuated or repetitious.
And, frankly, the majority of his fantasy, Moorcock wrote as a way to support what he considered his more serious work with New Worlds
, the New Wave movement, etc., as the magazine was always teetering on the brink of dissolution financially, and he did most of the supporting of it through his own writing -- which meant a lot done at a fast pace (hence some of the awkward wonkiness and serious slips now and again). This didn't prevent him from addressing his more serious concerns in the fantasy, and still producing some very good fantasy along the way... but it was written hastily and with little revision, hence is sometimes rather jumbled -- and always short (until well into the 1980s, at least, for the fantasy).