Reason 1: Water type HM moves.
Strictly speaking ever since Generation II, two or more Water type HM's have appeared in every game leading up to Generation VI.
Gold, Silver and Crystal had HM03 Surf, HM06 Whirlpool and HM07 Waterfall;
Ruby, Sapphire and Emerald had HM03 Surf, HM07 Waterfall and HM08 Dive;
Fire Red and Leaf Green had HM03 Surf and HM07 Waterfall;
Diamond, Pearl and Platinum had HM03 Surf and HM07 Waterfall;
Heart Gold and Soul Silver had HM03 Surf, HM05 Whirlpool and HM07 Waterfall;
Black, White, Black 2 and White 2 had HM03 Surf, HM05 Waterfall and HM06 Dive;
and X and Y had HM03 Surf and HM05 Waterfall.
Because you need Surf, Whirlpool, Waterfall or Dive to access particular routes, the majority of players will have at least one Water type Pokémon in their party.
Reason 2: Availability.
As you'll notice Surf and Waterfall have appeared in every single game since Generation II and there is a high number of Water type Pokémon that can learn both theses moves, this includes:
Squirtle, Wartortle, Blastoise, Psyduck, Golduck, Poliwag, Poliwhirl, Poliwrath, Tentacool, Tentacruel, Seel, Dewgong, Horsea, Seadra, Goldeen, Seaking, Staryu, Starmie, Gyarados, Lapras, Vaporeon, Omanyte, Omastar, Kabuto, Kabutops, Totodile, Croconaw, Feraligatr, Chinchou, Lanturn, Marill, Azumarill, Politoed, Wooper, Quagsire, Qwilfish, Remoraid, Octillery, Mantine, Kingdra, Suicune, Mudkip, Marshtomp, Swampert, Lombre, Ludicolo, Carvanha, Sharpedo, Wailmer, Wailord, Barboach, Whiscash, Corphish, Crawdaunt, Feebas, Milotic, Spheal, Sealeo, Walrein, Clamperl, Huntail, Gorebyss, Relicanth, Luvdisc, Kyogre, Piplup, Prinplup, Empoleon, Bibarel, Buizel, Floatzel, Gastrodon, Finneon, Lumineon, Mantyke, Phione, Manaphy, Arceus (Water), Oshawott, Dewott, Samurott, Panpour, Simipour, Basculin, Tirtouga, Carracosta, Frillish, Jellicent, Alomomola, Froakie, Frogadier, Greninja, Skrelp, Clauncher and Clawitzer.
By looking at the sheer amount of Pokémon that can learn both Surf and Waterfall above, you can see you've got a lot of choice to choose from and that's only from Water type Pokémon who can learn both Surf and Waterfall. Pokémon such as Pelipper might not be able to learn Surf and Waterfall but can learn Fly, Surf and Whirlpool. Many of these Water types you can get early to midway in the game and many of them can learn multiple HM's.
When you consider that a lot of the Pokémon I listed further up can also learn other HM moves such as Cut, Strength, Rock Smash, Whirlpool and Dive etc. You can start to see why Water types are considered such good HM Slaves. Bibarel in generally can learn all of the moves I just mentioned, plus Surf and Waterfall.
