Good question indeed.
Sri Krishna said
एतान्यपि तु कर्माणि सङ्गं त्यक्त्वा फलानि च।
कर्तव्यानीति मे पार्थ निश्िचतं मतमुत्तमम्।।18.6।।
Even these actions too must be performed by relinishing attachment and
fruits : This is my considered best opinion, O son of Prtha !
Sri Krishna also said, after delivering the entire BG, as follows:
इति ते ज्ञानमाख्यातं गुह्याद्गुह्यतरं मया।
विमृश्यैतदशेषेण यथेच्छसि तथा कुरु।।18.63।।
To you has been imparted by Me this knowledge , which is more secret
than any secret. Pondering over this as a whole, do as you like.
When Sri Krishna, who was a friend, relative, guide to Arjuna gave the Viswarupa Darshana, and delivered an elaborate lecture on Spiritual concepts, why should he said do as you like?
The secret/subtlety lies in the fact that Sri Krishna was explaining various issues to Arjuna, as a Guru.
A true guru like Sri Krishna, will never say that the disciples should follow his/her orders.
A true Guru will always shows the way to the disciple in a subtle and humble way and it is for the disciple to follow the route, toil and achieve the result.
The subtle message that Guru gives in being humble is in future, when the disciple gets the REALISATION, he should follow the suit and remain humble.
Sri Krishna is a TRUE guru.
When Sri Krishna said that it is my opinion, we have to remember that he was guiding in a humble way.