Why Bellas deserved to win the past 2 movies

They are the protagonists, and the protagonists are likely to prevail cliche (duh)

DSM sang both songs in minor keys (affects mood of audience)

The two leads are the only competent singers during their performance. If you compare the Bellas support to DSM's support, Bellas have more depth

The Bellas change the flow of their performance through the mood and tempo of their songs. First 2 are moderately paced, then flashlight is a much slower and brighter tone (tempo based on length of bass). They did that last movie too, going from Price Tag, softening to Beca's solo in Don't You, to Give Me Everything

The Bellas uplift the key of their song up a semitone. So what was already major is now even more major. More musicianship here.

I'm disregarding the 30 second clap sequence by the Bellas and the formations made by DSM because they are too irrelevant to a SINGING competition

So yeah, we knew they were going to win, but they legitimately gave a solid performance based upon multiple aspects of their songs. Bring substantial evidence to prove your point that DSM should've won, because there really wasn't much to it.