It's more of the Portuguese Empire collapsed otherwise they'd still be around till today.
FYI, the Arabs in question here were the Omani Sultanate who operated a gigantic slave trade on the East African coast.
Not commonly known, but the highest incidence of Arab genetic markers is in Yemen, followed closely by East Africa. Based only on genetics, an Ethiopian or Somali is much more Arab than an Egyptian, Syrian, or Iraqi. Although culturally speaking East Africa did not adopt/uphold Arabic language and culture as much as the Levant or Egypt.
