Very much this. Sight and Sound, along with a poll done at the Festival Mondial du Film et des Beaux-Arts de Belgique in 1951, were the first widespread instances/attempts to explicitly canonize a form of media. People didn't really conduct "best-of" polls/lists before cinema.
It takes much less time to watch a film than to read a novel, so any “expert” on film will have seen many more films than an analogous literary “expert.” All films except the very earliest examples were made in the 20th and 21st centuries. And book people are conservative