why there are only sentinel and scourge in this custom game called 'Dota'??

becoz the alliance and the hordes are playin' them, ya see

human - normal player who start playing Dota. it's the human spirit that counts
dwarf - they know how to get items...
gnome - good in escaping, and turn player into things that is shorter than them
night elf - he'll show up during night, or during necessary times

orc - who charges up recklessly into the gank crowd, but always getting away with kills
undead - human player that suffered during the war, now they can play 24/7
tauren - pumping hit points, it's their racial trait

lastly (a long one too)

troll -
composed 98% of Dota population that mostly self-proclaimed as 'PRO' (Play Rough Only); likes to carry torch around for flaming use (see aussie bush fire? it's their doing...);
words like 'PWNED', 'NOOB', 'L2P' are mostly used in game but they will easily get aggroed when such words addressed to them.

now the secret has been revealed...