For disk I/O, you have absolute control over the number and type of events that may be pending, allowing you to strictly bound resource consumption and defer handling of those events indefinitely with no consequences. Similarly, you can opportunistically use unused disk I/O capacity to bring future I/O events forward with little cost e.g. pre-fetching. The worst case scenario you have to deal with in terms of disk I/O is self-inflicted, which makes for relatively simple engineering.

Network I/O events, by contrast, are exogenous and unpredictable. You often have little control over the timing or the quantity of these events, and the only limit on potential resource consumption is the bandwidth of your network hardware. Not only do you have to handle the worst case scenario in these cases, you also have little control over what a worst case scenario can look like. This leads to very different design decisions and interactions with the I/O subsystem versus disk.