LM is special because is the only prestige class that fits any play stile. If weapon Master worked with bows or spells, their numbers would be higher.
Some reasons LM are so popular:
1) Languages
Do you want a character that can speak certain language?
A) You either spend an award, which you may not have or you may not be able to because the award was used for something else.
B) You either have to get a phrasebook and invest a large amount of time to learn it. (IF it is learnable and the build has enough Int for it)
C) You take the class that gives it (and most likely heavily nerf your build)
D) You take LM levels.
2) Arelith became a "You must have access to spells to buff yourself" (And also the existence of time stop / Mord scrolls)
LM giving access to UMD, having a good boost to Lore (+10 at level 5 and +5 from the feat requirement) means if any build wants to use lore 80 scrolls, this class gives access to that.
3) Gives casters a potential splash without nerfing their CL (and bard song)
4) Some classes do not have a high level cookie/incentive, allowing them to splash. (A.K.A fighters).
5) Loosing loot because it is locked? LM also covers chest skills.
Is it LM class itself? Or just all the other mechanics / current meta making this class shine?
At the end of the day, LM bonuses are more aligned towards QoL and RP stuff than "Every META Insta-win PVP must be this class" and gives access to mundane characters to tools to compete against casters.