The reason is that Tarkovsky saw film as an art as only a relatively recent phenomenon, with the early film-making forming only a prelude.
ðə ɹˈiːzən ɪz ðæt tɑːɹkˈɑːvski sˈɔː fˈɪlm æz ɐn ˈɑːɹt æz ˈoʊnli ɐ ɹˈɛlətˌɪvli ɹˈiːsənt fɪnˈɑːmɪnən, wɪððɪ ˈɜːli fˈɪlmmˌeɪkɪŋ fˈɔːɹmɪŋ ˈoʊnli ɐ pɹˈeɪluːd.