"Song" refers to the orality of the poem. This was "written" to be recited, probably with harp or lute accompaniment, which also explains the variations in the surviving nine manuscripts, the mysterious AOI that ends many of the stanzas, and the formulaic language. What AOI means is anyone's guess, but some scholars speculate that it might be a signal for the harp accompaniment or maybe a short chorus that the jongleur sang.
As for the formulaic style, yeah, it can get kind of old when a pagan is tossed a "spear's-length" from his horse for the tenth time, but how would you like to memorize 291 stanzas of old French verse? Cut the jongleurs some slack. These guys relied on repetitive phrases and simple images to keep the memorization manageable.
The interesting part about the title is that this song is of Roland. Why isn't it the Song of Charlemagne? You could make the case that he is the more prominent character. His emotions give the poem its human structure. His reactions and decisions shape the narrative. Even Roland's failure to blow the oliphant can be traced back to old Charlemagne himself, who received a clear-as-glass news bulletin dream from God that Ganelon was up to no good. He could have stopped everything. Although he says he can't know Marsile's mind, Charlemagne actually knows a lot—he just doesn't put it to good use.
This failure to act on knowledge ultimately contributes to the slippery slope leading to Roland's tragedy. And that's the real reason this is Roland's song. It's his tragedy. He is the one who suffers the most. It's his tragedy that leads to the eventual taking of Saragossa and the securing of the Frankish empire.