because its over an optical toslink connection, youre limited to stereo pcm encoding (this same thing also holds true for coaxial), UNLESS the encoded format being output is something other than stereo pcm (dvds for example, typically use ac-3 encoding, aka dolby digital encoding for use over a digital connection, which allows for up to 5.1 seperate channels for the basic surround), but if you dont have that encoding, or something similar to that, then all youll get is 2 discrete channels (regardless of your systems speaker settings otherwise). a workaround for that is to have something like dolby prologic, dts:neo, or various dsp modes on your digital decoder, such as a HT receiver, to simulate surround sound to your other speakers, with only 2 front channels being received.
surround analog offers more of the same if its only 2 discrete channels, cmss and eax for example are also workarounds to getting matrixed surround with only 2 channel stereo.
another option instead of being limited to only dvds (and such) for surround playback over a digital connection, is to look for a sound card that features dolby digital live (DDL), or dts interactive/connect (DTSC). a card with either of those features will re-encode your sound (in realtime) into a compatible surround format (such as dts or ac-3), so then you can get the surround youre looking for, up to 5.1 surround by default. a sound card with an hdmi connection, will allow for hd surround up to 7.1+ channels of uncompressed pcm, but if the sound card doesnt support an hdmi output (dolby digital plus is an example of a discrete 7.1+ channel surround format, such as on blu-ray and hd-dvd discs). otherwise then youre limited to 5.1 compressed surround (standard ac-3/dts, due to coaxial/optical not having enough bandwidth for more, and even standard video dvds simply not having enough capacity (with both video and audio on it)... 6.1 is possible though over coaxial/optical, but only if the surround format is encoded for it, such as with dolby digital ex or dts-es)