I think a bigger reason for lack of online back issues is that very little, if any, of the typical magazine content has any time-sensitive relevance to the average reader.
So most people would just read the back issues.
Let's face it, you could always read yesterday's papers. It wouldn't actually make any difference to you 99.9% of the time.
In reality, almost nobody wants to read a magazine on a PC, so the circulation would plummet. These mags aren't cheap!!