Why do people tend to reply to only one topic when multiple topics are brought up simultaneously? This is a situation more frequent in online conversations, where you can type many things before the other person has a chance to reply.

Perhaps cognitive load, multi-tasking ability, have something to do with it? At any rate, it does seem like the person "ignores" the other unanswered question at times.
I believe it has to do with two main factors:

1) Emotional response to one of the topics

Consider the case where a webpage or paragraph of text contains multiple topics. Each one with different emotional response from the reader:


Topic 1: X____|
Topic 2: XX___|
Topic 3: X____|
Topic 4: XXXXXXXXX
Topic 5: XX___|


Where the number of X's is how strong the response is. Suppose 5X's are needed to bother responding. Assuming the topics are encountered sequentially, Topic 4 will activate the strongest emotional response. Since it's also above 5X, the person may start responding, leaving topic 5 unread and un-responded to.

2) Social/technical norms of the online communication system

In many web platforms, the main means of commenting/replying is via typed text boxes, usually with minimum and maximum length limits. That's the technical norm. The lower the limit (i.e. a tweet), the less likely more than one topic will be responded to. Expectation of interruptions could also play a role. For example, in an online chat, many other emotional topics may be mentioned while typing out a long, nuanced response, potentially stopping it from being finished.

The social norms come into play when a reader observes how others have responded to the item and in what format. If others responded with short one-liners, the reader may be inclined to respond with a one-liner. If others wrote up elaborate essays as responses, perhaps the reader may do so as well.