Each of our rabbis is in good standing with their movement. They adhere to what their movement requires of them. In some cases that means doing conversions without these halachic requirements. In other cases, rabbis are more strict.
Just as each student is different, so is each rabbi and their interpretation of what Judaism requires of a giyur (convert). More traditional rabbis will require mikvah, bet din, brit milah in the case of men, as well as conversion of spouses and children. Other rabbis who are more liberal on halacha will not require some or all of these.
Students are encouraged to make their own decision about which rabbi to work with in the most educated way possible and to consider any implications that conversion without these traditional requirements may have.