Online visual studio is running in the browser, and not hosted by node. This means an extension must support running in the browser sandbox and many node features like file I/o are missing. Actually the extension can perform file I/O via the browser api, but it must be rewritten to use it rather than the node version.
Another restriction is the inability to load native libraries.
Over time, if the author of the extension is interested, more extensions will be supported by vs online.