Why was Aqidah called the science of Tawhid, even though Tawhid is only a part of Aqidah?

The science specializing in the study of 'aqeedah (creed) was named 'Tawheed' (monotheism) because Tawheed is its greatest subject. It encompasses the Oneness of Allah, the conditions for realizing it, its implications, and its nullifiers, as well as other matters such as proving prophethoods, scriptures, divine decree, the Last Day, angels, jinn, and devils.

The naming of sciences is a conventional designation. The early scholars referred to this science as 'Sunnah,' 'Sharee'ah,' and 'Iman.' Scholars of Kalam (speculative theology) called it 'Usul al-Din' (Foundations of Religion), and eventually, the name 'Ilm al-'Aqeedah' (Science of Creed) became established.