I find this frustrating also. I did buy Season 5 on DVD. The other seasons are available for download on Amazon, but only within the US, so they are inaccessible to those of us fans who live elsewhere.
I just realized this has already been discussed before. Thanks for the replies. I still just don't understand why they don't release them on DVD! I would so buy them. I would loooooove to see the first couple of seasons again.