It's not just about giving Ashley a romantic role; they actually gave her a real character arc that she barely had in the original. Leon, still carrying the trauma of Raccoon City and his distrust of Ada after she burned him, finds something different with Ashley.
Why did Capcom go and add all that romantic stuff with Ashley in the RE4 remake, but then intentionally tone down Ada's side? They cut Leon's iconic line from the original – 'She's like a part of me I can't let go' – and replaced it with him telling Ada 'we're on separate paths.'