I suspect it's simply the newness of the medium.
Back in the old days, I'll bet a lot of folks argued over whether those new-fangled moving-picture-shows could really be considered an art form - but few would doubt it now (in spite of the prevalence of crap in the medium, to this day).
Whatever it is, I believe it's art, if it can stir your emotions - not very objective, I know. I remember clearly the first time I thought that way about a video game - it was in FF7, when Aeris dies. I was a little in awe, that a game could get under my skin like that.
If anything can pull you in and mess with you emotionally like that, how can you say it's NOT an art form?