The decline in monthly print medium is universal, mostly due to a loss of advertisers. Once advertisers leave, magazines and newspapers have to start cutting costs which reduces the amount and quality of the content included. The feedback cycle continues until there's nothing left. In the 1980s magazines were the primary medium through which information about new technology products spread. Then in the early 1990s people began moving online to the internet and the world changed.
In Byte's case specifically the large space devoted to ads for mail order services started to decline significantly in the 1990s. In part it was a change in the kind of reader that was interested in computers. There was no longer a need to publish the price of CPUs, SRAM and other ICs in the back of Byte as that wasn't what people were buying. Plus the mail order houses had built up their own lists of customers by then, and would directly mail flyers and catalogues. Computers were no longer easily built from scratch as 32 bit CPUs became more complex and out of reach of most hobbiests.
I loved Byte magazine in the 1980s, and learned so much from it... The monthly hardware project from Steve Circia was fascinating, and there were articles about data structures, languages and even filesystems. I am sad for the loss of that enjoyable monthly experience.