[P]resent only one idea at a time.

Most posts do present one idea at a time. However it may not seem like it because most of the ideas presented are additive - that is, you have to have a fairly good background on topics that have been presented previously in order to understand the current topic. OB and LW are hard to get into for the uninitiated.

To provide more background and context, with the necessarily larger numbers of ideas being presented, while still getting useful feedback from readers.

That is what the sequences were designed to do - give the background needed.