Roman rule revolved around cities and agricultural estates along the littoral strip of the Maghrib. The Berbers thanks to the camel were able to move out beyond the "limes".

I should start out by saying that there still exists a fair number of Imazighen (Berbers) today, so the idea of complete Arabization isn't correct. However Arabic cultural influence was likely increased due to the following factors:

• Cultural similarities: Both were clan-based nomadic cultures.

• Unity derived from Conquest: The Berbers were a vital component of the Arab armies that crossed Gibraltar into Spain. Gibraltar is after all named after a Berber.

• The impact of the Almoravids/Almohads: The Romans simply did not have a Berber group dedicated to proselytizing, nor did they rule as vast an area containing Berbers, as these two did. Tangentially related would be the effect of the traveling Shiite missionaries on the Berbers.

No, the failure of the Romans has nothing to do with the current situation, the Arabs simply followed a different policy, aided by the allure of Islam, and the intensity of the inter-Islamic ideological conflict which occurred in the Maghreb.

Source: General History of Africa Vol III, M. Fasi(ed). Chapter 9.