Cricket is an English game and was played by the Lords and Roylaties in earlier times. It was more an outing on a Sunny day. It was a gentleman's game and if anybody got out, he used to walk. Players, usually from Royal background or belonging to the upper class of England, preserved their dignity and believed in playing a a fair game and never resorted to abusing, pushing, pulling or showing disrespect to any player. They also applauded a good show of the opponenet and even congratulated them for taking a wicket or scoring a shot. It was normal and expected that if the umpire was unable to see something then the player would honestly reveal the facts which would affect a decision. Thus it was a perfect gentleman's game(and it still is, despite the "gamesmanship" replacing "sportsmanship" in the professional cricketers).
Thus whenever it was something unfair and having some element of injustice, it was called "this is not cricket".
However, with passage of time and money and professionalism entering the game of cricket, players are not maintaining the highest standards and resort to sledging(using abusive language), giving statements in press to distract the opponent, taking money from match fixers etc. have maligned the image of Cricket.
It is surely not cricket.