Because this is the methodology of Allah, He who legislated it for all of the prophets (alayhimus salaatu was salaam). There was not a Messenger who called his nation, except that he began with Tawheed, even though the call of the prophets comprised every good for mankind.
Just as the messenger (Sallallaahu alaihi wa sallam) said:
“Indeed there was not a Prophet before me, except it was a must that he guides his nation to all good, which he knew of for them, and warns them of all evil that he knew of for them. (1)
Therefore, the Prophets (alayhimus salaatu was salaam) related every good to mankind and (related) everything that aids them, however they began with the most important, then the most important (after that).
And the one who contemplates on the Quran, he sees that the call of the Prophets share very tremendous principles.
From them: At Tawheed, and from them Affirmation of prophethood and Affirmation of the resurrection and accountability.
However the core basis of their call – that which the conflict between them and their nations revolved around – was indeed only Tawheed, and specifically Tawheed of worship.
Because Tawheed of Lordship and Tawheed of the Names and Attributes, you do not see in the Quran any differing between a Prophet and his people in anything from this (Tawheed of Lordship & Tawheed of the Names and Attributes of Allaah ).
They denied the resurrection, no doubt, and they rejected it, however the most severe (thing) which they denied was the call to sincerity in the religion for Allah alone. So you see the call of all of the Prophets, as Allah has elucidated in His book, the first thing they began with was: The rectification of the Aqeedah, rectification of that which is deficient in the issue of Tawheed of worship.
At Tawheed Awwalan pages 4-5 by Shaykh Rabee bin Haadi Al Madkhalee