Why is stalemate a draw? It is so difficult to force stalemate and sometimes unintentional too, so why is stalemate a draw? 


  As the opponent doesn't have a further move, why is it not considered a win?
Stalemate is a draw in classical chess yet there are other chess variants both historical and modern where stalemate is not a draw. 

Very early versions of Chess, such as Shatranj Chess (props to Andrew Latham) declare the player causing stalemate the winner and even today there are callings to return to that rule.

for example: 

Larry Kaufman Chess Life Sept. 2009 writes: 


  "calling stalemate a draw is totally illogical, since it represents
  the ultimate zugzwang, where any move would get your king taken"


A typical counter to this point is that Chess tradition dictates that a stalemate cannot result in a win for the stalemating player because it would require a suicidal move from the stalemated player (moving your king into check is, in most forms of chess, an illegal move). If there are no moves on the board that you are able to make (other then resigning) then in making the stalemating move your opponent has effectively ended the game without a definitive result (capture of the king) and is judged to have not won the game.

So.. "In Classical Chess what is the probable origin of stalemate being considered a draw?"

The earliest reference I can find to Stalemate being a draw in formalised chess rules is 13th century Italy where the version of Chess played declared that a game could only be won by checkmate or resignation. Stalemate, under those terms, describes a state where a player cannot make a legal move. In effect the game has stalled in a state where neither player can achieve Checkmate and is drawn.

Stalemate as a draw then spread to Germany (1400s), Spain (1600s) and throughout Europe by 19th Century when the formal rules of what we know as classical chess were put in place. 

England was one of the final adopters of Stalemate as a draw in the 1800s (up until that point the stalemated player had actually been declared the winner).

Opinion:

Yes tradition does favors the player in stalemate but it has been that way for a very long time. The flipping of that advantage to the stalemating player would greatly change the nature of the endgame (and thus the game of chess and a huge amount of chess theory), which is probably why such arguments have not recieved too much traction.