Mathematics of 6/49 game. Why is it not 5/48 or 7/50?

The mathematics of lottery is based on 6/49 game. Each player selects 6 numbers without replacement from an urn of 49. If the 6 numbers match with the lottery number then it is a jackpot. The probability of it happening is 1 in C(49,6) or 1 in 13 million. I have two questions: [1] Why does the urn consists of 49 numbers and not 48 or 50 numbers? [2] Why one has to select 6 numbers and not 5 or 7?

Even if the numbers change to 5 selections from 48, the odds 1 in ~2 million. Is it too bad for a lottery?