pre-noun adjectival
chief; leading; prominent; principal; main; important
Pre-noun adjectival (rentaishi) used to describe the most important or prominent members of a group, or the main points of a topic. Often used with nouns like 人, メンバー, 理由, 作品.
主だったメンバーが会議に出席した。
The leading members attended the meeting.
主
There are three main reasons.
主要 is a noun or na-adjective meaning 'major' or 'principal', used in a wider range of contexts. 主だった is specifically a pre-noun adjectival form, more limited in use and slightly more formal or literary.
Derived from the adjective 主 (おも, 'main') combined with the past tense form of the verb 立つ (たつ, 'to stand'), literally 'that which has stood out as main'. The form 主だった functions as a fixed pre-noun adjectival.