She wants to foster a broad coalition of support from all the flavors of Democrats and moderates, including the (mostly) older demographic who see bus/bike lanes as radical and liberal and the (often) Black population who feel like white and Asian liberals take the their votes for granted.
To put another spin on your analogy: ignoring the concerns of elected Black leaders and installing anti-car infrastructure in Roxbury and Mattapan could absolutely matter to Dem primary voters in South Carolina.
Bike and bus infrastructure is loved by urbanists in Boston, but "we" are extreme weirdos nationally. The voters Dems need on the national stage drive trucks and SUVs and think that stuff like center-running bus lanes is fringe and "liberal elite" coded. It's not worth her spending her political capital on it in this national-focused context. And the same goes for White Stadium. Building a new city high school stadium through a public-private partnership with a pro team sounds like the cool sort of innovative idea that people want in government. Remember that in most parts of this country, governments pay pro sports teams to come to town, while here it's the exact opposite. The "Franklin Park Defenders" read nationally as weirdo woke tree-huggers separated from reality.
I'm not saying I agree with this thinking, but I think it's how she's angling. She sees Elizabeth Warren's image (loved by the white educated urban left while never being able to get traction among the national party more broadly) and doesn't want to be that.
On the national stage, stories and photos of her standing with Culpepper and Worrell to protect parking spots could help her.