Here's why:
* Bleach is finished: The manga and anime are complete, meaning there are no plans for new canon content.
* Subtle hints: While the series heavily implies their affection, it never explicitly states it. They share a deep bond, playful banter, and clear trust in each other, but their relationship remains ambiguous.
* Kubo's intentions: The creator, Tite Kubo, has never explicitly confirmed or denied a romantic relationship between them. This leaves it open to interpretation.
* Fan interpretation: Many fans enjoy interpreting their relationship as romantic, drawing on the subtle hints and their long history together.
Ultimately, whether you see Yoruichi and Kisuke as romantically involved is a matter of personal interpretation. There's no definitive answer, and the series leaves it open to the imagination.