Because it's only this road with 〜だけ
Grammar Explanation: Limiting with 〜だけ

Takagi convinces Nishikata to spend his summer practicing riding his bike with her on the back.
One day, she asks Nishikata if he’s progressed on his summer homework, saying they should go to his house to work on it.
Since the road back is wide and without cars, Takagi convinces Nishikata to ride doubles to his house.
- 西片:
- 「こ…この道
( だけだからね。」- “Because it's only this road, okay?”
Nishikata says he’s riding doubles because it’s limited to 「この道
Key Points
だけ = only
Here だけ limits what Nishikata is agreeing to.
He is saying the double-riding is okay only for this stretch.
Why だから appears at the end
だから marks the reason.
The meaning is “I’m only doing it because it’s just this road.”
ね seeks understanding
The ending ね sounds like he wants Takagi to accept that limit.
It gives the line a defensive “okay?” nuance rather than a firm declaration.