1. People tend to skim and a question could be lost even in a two-sentence paragraph.
2. Email's structure means people tend to reply "at the tip" and branching conversations are difficult to understand.
Contributing to this latter problem are:
a. SMTP (to/cc are too flexible, each message is it's own "thing")
b. POP (deep conversations are just a stack of messages some people may not have the "original" and can't easily reply higher up the tree without breaking client threading)
c. Email client visualization of message threads are generally bad. I haven't seen a single client do this well. Outlook can, but out of the box has a very "flat" view.
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So, people tend to read at the bottom and if someone missed something early in a thread you have no chance of getting it addressed a few messages in.
IMO something more akin to newsgroups or even reddit/HN tree-view threads could be a better fit for business discussions, but I haven't seen anybody try it.