Why do Muslims celebrate muharram

 slashing is not a shia tradition,it was entered among shias after Mongolian invasion(hurting your body is forbidden in islam)

As Maddykinzzz said Not many Shia Muslims 'slash themselves'.It is haram according to shia schloars' fitwa.

We dont celebrate this day because it is the day in which Hussein(the grandson of prophet) was martyred.

Ashoura(which means the 10th day of month in Hijri calender) was a celebration among Jews and they were celebrating it and fasting on it because they thought it was the day in which the God saved Israel tribe by drowning Pharaoh in Nile.
Muhammad(PBUH&HF) ordered muslims to fast on Ashoura day and this fasting was obligatory until the order of fasting on Ramadan.After obligation of fasting on Ramadan month,the order for fasting on Ashura day was cancelled.

First of all not all muslims do as it is not a huge festival or anything like that.
any way first of all muharram means the first month of the islamic calendar 
and it is one of the  four sacred months of the year and it means forrbidden 
and sinful . the reason people celebrate this by  not eating or drinking  so like fasting . and there are ten days for this . some fast during this day because there is something that is like a law   but u don't have to do it and this law is called a hadith and that moses ( a phophet ) and his pepole obtained a vitory over the 
egiption pharaoh on the tenth day of muharram and muslims take this extremely importantly that is why mostly beacause it is a hadith meaning something u are expected to do 
hoped this helped thanks please can u make my answer the brainlist as  i spent a lot of time writing this thanks