Why was there so much of corruption during president grants presidency?

Unfortunately, just because someone was a famous general (as Grant was) does not necessarily make the person an effective president. Ulysses S. Grant seems to have hired too many friends and relatives, rather than selecting people who were qualified for the positions he assigned to them. Patronage (giving jobs in the government to big donors or big supporters, whether they have the right experience or not) has been a long-time problem in government, and Grant's administration certainly exemplified it.