chionos has a point. think about it like this:
theres only 6 spots in a party right.

-you know you NEED a healer, so a Whm is essential.
-you know you NEED a tank, the only 3 options are PLD, NiN, or Warrior (SAM cannot tank, they just are not made for it.)
-your gonna want someone with some kind of magic, usually a BLM, but SMN, RDM, even BRD is possible.

-your gonna want the remaining 3 spots to balance out whatever you have so far. if you have a BLM then you might want a RDM or BRD to refresh your WHM & BLM. if you have a BRD already you might be able to get 3 Melee.

Either Way you have a lot to choose from at this point: THF, MNK, SAM, DRK, DRG

so what happens is there is just SOOO many Melee to chose from and theres usually only 2-3 spots in a party for melee, that many melee find it hard to get groups.

SAM isnt any harder to get a group than anything else, in fact its probably much easier than being a high lvl DRG or WAR.