In short: Without the pipeline, railroad capacity will grow, overall safety will decline, and economic costs will be higher. Who would benefit from that outcome?
These realities demonstrate that the opposition to Keystone XL is almost wholly ideological, driven by a quasi-religious abhorrence of fossil fuels and the wealth, human wellbeing, and independence from government coercion that they facilitate.
Perhaps a journalist might ask Biden to explain his reasoning, and how that rationale was informed, or not, by the truths summarized above.