The same reason Iranians, Afghanis and Turks are primarily Muslim and close to the the Gulf Arabs yet still speak their native language.

Also, Somalis are proud of their culture & heritage and tend to be extremely ethnocentric. Mixed Somali children from significant Somali communities will most likely be raised as Somali no matter if one parent is Arab or Farsi or White or whatever.

But all the Sultanates and the states of Somalis throughout the past 1400 years in Somali history have used Arabic along side Somali to rule their people so Arabic was more for the Religious and ruling elites and not for the regular people.