Why were some delegates opposed to having a president to lead the executive branch

Those opposed were anti-federalists. This meant that they opposed the federal government having too much power. They though the president being in control would give the federal government too much power.

The delegates were not sure of a President having most of the powers in government. In fact, they thought of dividing the power to three. They also thought that communication and information gathering was too slow. Lastly, the fear of having the majority of votes.