While Narayana is a means to attain Him, then why did the Gopis of Ayarpadi and Andal observe Katyayani vratam to attain Kannan?

Gopis observed Katyayani vrata as it was their tradition; they observed it to get a proper alliance. They did what they knew.
Andal did not observe Katyayani vratam; she just accepted their action, stated ‘nArAyaNanE nammakE paRai tharuvAn’, and observed vratam; she emoted or portrayed the doings of the Gopikas.
Andal’s means and goal was Emperuman alone. She followed the austerity of Gopikas and did not observe vratam towards a demigod but towards Emperuman alone. Gopikas simply followed their tradition.