Easy-peasy: Bowlers like to rub the "ball" (round, hard, red/white object) in long patches on their pants, and using the crotch area would be uncomfortable.
Also, batsmen have to be weary of the above mentioned "ball", and while thigh pads and leg pads cover the area quite well, without pants, they would look like Indian freedom fighters from the 1930s-40s, hoping to escape the police lathi charges.
Thirdly, and this is primarily for subcontinental matches, the pants save the players from mosquito bites.
Of course, MCC's answer might differ, in which case I will apologize for this.