Why tattoos are haram when male circumcision is halal?

 Allah has created the human body in the most beautiful way.  Allah has forbidden it to be distorted. Therefore, Islam forbids tattooing on body parts. This is because tattooing distorts the human body and unnecessarily causes harm.  There are various hadiths also telling us that it causes the wrath of Allah.

 ابن عباس قال والمستوصلة والمتنمصة والمستوشمة من
 (ابو داود -4170)
 عبد الله الله لعن الله الواشمة والمستوشمات للحسن للحسن خلق

Allah, Who has created the human body in the most beautiful way, is also fully aware of it's nature. Allah has made circumcision compulsory.
Circumcision is an act practiced by the Prophets and commanded by our beloved Prophet (peace and blessings of Allah be upon him) to follow them.

 ابو رضي عن عن الله الله عليه الفطره الختان: الخ (الخ)
 هريره الله عنه قال قال رسول الله الله عليه وسلم اختتن النبي صلى عليه وسلم ثمانين سنه (رواه البخاري ومسلم)
 تعالى: ان

Modern science also encourages circumcision. If not circumcised, the genitals are more prone to accumulate impurities under the skin and cause many diseases.

Circumcision is therefore obligatory for every Muslim.  But a tattoo is a disfigurement of the human body unnecessarily and causing harm.
Hence, it is impermissible.