Good question! I feel like this is partially because there are more people in the fandom who like Mordred than Lancelot, which means that people tend to focus on Mordred more (see this old survey!). The second reason might be that Mordred and Galahad are more well known than Galehaut, and Lancelot is mostly known around Arthur and Guinevere, which leads many people to ship these three characters (as ot3 or just Guinevere/Lancelot or similar). In general, Guinevere/Lancelot is the most popular pairing in media, which I suppose tends to overshadow Galehaut.
For specific reason about Mordred/Galahad, personally, I like the “two faces of the same coin” element. Both bastards, both not recognized, both with clearly troubled relationships with their fathers, both sons of mothers who tricked/shouldn’t have been with their fathers. I would say both sons of sin (but I suppose it’s debatable) and yet they end up on completely opposite sides. They both have quests that will kill them at the end. It’s intriguing!
Arthurian Legends 