Why the ending is a tie？

Are you talking about a game of yours? if you do, then insert it here to analyze it.

Oh i found it on your profile. Here it is:

Because it was a stalemate. Your opponent is not in check but has no legal moves.

That's a draw.

You could have easily checkmated your opponent many moves earlier.  

Click on analyze if you want to see it, other people of this forum

Yes, as @justbefair said, It is a stalemate because the enemy king cannot move. Your queen is covering f7, f8, h7 and the dark bishop is covering g7 and h8. So the king cannot move anywhere.

Be careful in endgames when your opponent has only the king because you might stalemate him.

To avoid this try making every move a check, until you checkmate the king or try to leave some pawns to the opponent so he can move them instead of the king

yyes that

And just to avoid anybody having a common beginner's misunderstanding: in this particular position the King is Black's only piece so with the King having no legal moves and was not in check that means it is a stalemate.  The actual rule says that there are no legal moves by any piece and the King is not in check. IF Black still had a pawn on c6 then that pawn would have a legal move and it would not be a stalemate even though the Black King cannot move (I work a lot of scholastic tournaments and many kids have made that mistake).