What causes people to write compound words as distinct words?
In many Germanic and Finno-ugric languages there are many compound words. One does not write "yhdys sana", but rather "yhdyssana". Learning to write these correctly is notoriously hard for people, at least in Finland and Norway; there are mistakes everywhere, even in the simple and systematic cases, like compound words created from two nouns with nothing fancy happening.
Often it is the influence of English that gets the blame. However, I vaguely recall reading arguments to the contrary, probably based on chronology; the problems were present before English was as dominating as it is today.
What causes people to write compound words as distinct words? Is it the influence of English or is there some other significant cause, too?