Samuel was likely unaware of the man of God who previously came to speak to Eli (1 Samuel 2:27),1 so he did not know that Eli was already aware of a coming judgment. Added to that, Eli was the high priest and his mentor. They likely had a good relationship, and with Samuel still being very young, he would be reluctant to convey a message that would hurt Eli.
15 Samuel lay until morning; then he opened the doors of the house of the LORD. And Samuel was afraid to tell the vision to Eli.