Upagupta was a Buddhist monk who used to live a simple and saintly life based on his philosophical and spiritual faith. So, he did not need the luxury of a home which Vasavadatta was offering him.

Moreover, Upagupta believed in selfless service to the mankind. He was ready to offer help whenever someone needed it. He thought that Vasavadatta did not need his service at that moment. So, he promised to visit her when the appropriate time would come.

So, Upagupta’s response was aligned with his spiritual faith and wisdom he had gained as a monk. It’s so natural that a hermit like him won’t be tempted to promptly accept the invitation of a young girl, however beautiful she might be.