she knows better than to ask him to stay.
The likeliest and easiest excuse to offer that she can't manage to ever say those words out loud would be that of her pride—she's still suspended in that awkward transition from child to adult, and despite her un-volatile and calm by default nature, it's not a stretch to assume that she's still susceptible to that particular brand of petty feelings, especially when her status (champion) is factored into the equation.
But with Jun, things never seem to fall into the probable realm of predictability or simplicity. Sometimes she wishes it were so, but then, Jun wouldn't be Jun (and she wouldn't be so heartbroken, but all things come with a price).
She remembers starting her journey with variable degrees of uncertainty, when Jun pushed her towards Futaba's neighboring lake with a gleam of mischief in his amber eyes. His effervescent and nearly infective excitement was almost tangible in contrast with her muted countenance and her unsure posture, a mixture of exasperation and long-suffering, friendly affection. Jun was always that way, in their childhood—dragging her along with him on all his expeditions, all his futile searches for fairytales nestled away around their sleepy little hometown. At the time it was almost a nuisance, but now, she misses those days dearly.
He still crashes into her, she's still sent sprawling, he still never helps her up. And each encounter ends with the sight of his back when he leaves at that rushed pace, and her throat closes off with the effort of withholding her desire to tell him "Don't go." She can't.
To ask him to stay would be selfish; they're past that stage now, they can hardly be considered friends anymore. She can never get him to slow down, and he won't be content until he surpasses her. It's that goal of his that is so evident, she knows that Jun can't stand how she ended up being the hero when it was supposed to be him, when his father's legacy is weighing down on his shoulders.
Yet she wants him to take her by the hand again and bring her back to a time when their friendship was an unquestionable, unshakable thing; before it was marred by their vastly different paths and an unrequited jealousy.
(Before she started struggling to keep up with him in a physical sense, and he started struggling to keep up with her in a metaphorical one.)