Glorantha is weird combined with the fact D&D was first and good enough.

Drawing a maze with room on a piece of graph paper combined with the simplicity of D&D stats is several order of magnitude easier than anything Runequest had or could do. And this was largely true of many D&D competitors.

With the notable exception of Tunnels and Trolls, most of the RPGs that tried to compete directly with D&D tried go head to head by offering more details. More "realistic combat", more character option, etc. What this did was made their game more complex and less appealing to the novice. About the only game I recall, that came close to D&D in simplicity yet was distinctly different was Melee and Wizardry however that had a history of being a wargame first.