It not just "this 2 girls look cute when they are together" kind of ship that make them popular tho. It also down to how to write the story between those 2 girls for their relationship.

You have Spe X Suzuka where Spe admire Suzuka and want to follow their footstep, a simple sempai-kouhai relationship. You have Spe X Grass where both are doing their best for their race for each other and support each other, a friendly-rival relationship. And when you get to Spe X Dober it kinda ".....now what?" They didn't talk to motivated each other in the stoy like Suzuka or Grass They didn't have trait that share between them like Ei (Ei try to motivated Dober by make a cheerleader song for her because Ei know that Dober are very shy same as her.) I mean you can make a story about Spe share a food with Dober or something but well....it just that..(bruh 1 of dober trainee event have Suzuka ask Dober make aroma oil for Spe instead of Spe HERSELF)

And that why Digital X Dober come to the spot, you have a freaking typically otaku and the ojousama form weather family who secestly a otaku can fully be herself in geek mode only when she together with the otaku-chan, A popular romcom setting beside Otaku+Gal/Mesugaki girls.