 It was exactly what Umar wanted, as he is quoted saying 

The Byzantine empire already exhausted after major defeats in Yarmouk and Northern Syria was left vulnerable to Muslims attacks and its very existence in Anatolia was threatened.Umar, apparently was not interested in occupation of Anatolia, it had a cold and mountainous terrain with no economic incentives, soon after the occupation of Byzantine Armenia, the time when chaos was at its peak in Byzantium, Umar had already rejected Khalid and Abu Ubaidah's proposal of invading Anatolia.More over Umar, due to his strong desire to consolidate his rule in the conquered land and owing to his non-offensive policy left the remaining Byzantine empire on its own.