this is just my opinion.

I feel like there was ten times more chemistry on screen with grant and naya then there has ever been with naya and heather. Maybe heather isn’t as developed as a actor or something. I’m not sure. But naya gives everything to her brittana storyline and heather barely returns it. Even in the hurt locker speech when she said she loved her, it was only really emotional to the hardcore brittana fans. As a fan of the show, who likes Brittana but isn’t emotionally invested in their relationship, I can honestly say that their appeal does not reach because of the on screen chemistry of heather and naya. I wish it did. I wish I could want them to be together the way I want Finn and Rachel or Puck and Quinn or even though I’m not a fan of the relationship itself, Kurt and Blaine. Chris and Darren have enough on screen chemistry between them that they transcend the words on the script written out for them. And even fans of other ships like Kum or Kurtofsky (unless they passionately hate Klaine, which is true for some) can see their chemistry. I feel like in one two minute song in the first episode they’ve ever had scenes together in, Grant and Naya transcended their characters. 

Just my opinion.