For me it was the content to ad ratio. So many of these magazines are filled to the brim with ads, and have very little real content. Why would I spend money to look at a bunch of ads?
Before all the e-tailers existed one would often buy a computer magazine such as Computer Shopper for the ads and the articles were almost a nuisance. True enough now that the best deals are all online though.
I still have a box of them somewhere, I have one from early 90's that still has all the circles on vendor ads where I was doing price comparisons. Those were the days.