Why were seasons 2 and 3 saved, but season 1 wasn't?

The concept of "classic TV" didn't exist in those days, and once a series was finished, it was pretty much regarded as being of no further worth. So to save money, many TV networks and stations erased the old tapes to be reused. Sometime in the Sixties and Seventies, they started realizing how wrongheaded that policy was, but by then a lot of early stuff was lost, including episodes of Doctor Who, The Tonight Show with Johnny Carson (a fact Carson mentioned many times on his retrospective clip shows), and sometimes a whole series. All of Jack Paar's Tonight Show episodes are gone, for instance, and we only have Carson's from 1972 forward.

As for The Avengers, here's a quote from Wikipedia's article:

Most of the archives of two ITV contractors Associated British Corporation (ABC) and Associated-Rediffusion were destroyed in the 1970s after they were merged to become Thames Television. Associated-Rediffusion's archive suffered considerably more damage than Associated British Corporation's, leaving little of No Hiding Place, The Rat Catchers, and other programmes. Almost all of the entire first series of The Avengers was erased shortly after transmission. (no citation given)

And here's the full article:

http://en.wikipedia.org/wiki/List_of_lost_television_broadcasts

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"Oh, well" said Zanoni, "to pour pure water in the muddy well does but disturb the mud !"