"divx streaming" or "divx hoster" (as opposed to flash streaming & hosters) are just shortcuts to describe the state of converting and method of streaming - if a particular server converts, its called "flash hoster" and if it does not, its considered a "divx hoster".
while divx & mkv can be streamed, it does not have flash capabilities, unlike mp4 (properly converted) and flv.
that a means person streaming divx/mkv does require plugins for most browsers, either vlc video plugin, alternate or container specific plugins, instead of just streaming right away with flash, which can be expected on pretty much any computer already.
now the reason why people stream at all is convenience, and that in most cases means not having to download or install anything new at all. (following this train of thought, i could even argue that the welcoming screen of datacloud, offering to "regular download" might on itself be responsible for some bounce rate, as opposed to lets say "continue free")
just to fill this rant to full extent of refference
- flv has streaming & seeking abilies, but its rather large
- mp4 has only streaming abilities and uses superior x264 codec, making it the smallest for its quality. its widely preffered converting choice amongst flash hosters.
- mkv cant be flash-streamed, but its still converted with x264, meaning its conversion, if properly done, eg. plain demuxing instead of converting, can leave no strain on the servers. (on the other hand, if converted, can result in some pretty monstrous inbread freaks of nature)
- divx... well, at least it has some nostalgy value in it.