Why is the Bifrost depicted as being the only way of transporting to the other realms? We've seen quite a few examples that prove otherwise.
The Bifrost plays a big role in the first and third Thor films, and as such there is a lot of weight to its power and those who have access to it. The Bifrost is the bridge that allows transport between the nine realms, which might make it seem like
it is of the utmost importance and is capable of power that cannot be replicated, but this isn't exactly true. In fact, the Bifrost basically lost all importance the second Thor showed up in The Avengers. Heck, we could even go back farther and say that it lost importance the second Odin used his spear to open the bifrost, since it proved that Heimdall's sword wasn't the only key.
And yet, Hela sought out Heimdall's sword as the key, despite the fact that she knew her father's Spear would have the same effect. Furthermore, why is the Bifrost depicted as being the only way of transporting to the other realms? We've seen quite a few examples that prove otherwise, one being the Tesseract, which Loki and Thor used to get home at the end of The Avengers and when Thor showed up to Earth using Stormbreaker in Infinity War. This leaves us with a mystery, both in the sense of what the Bifrost is and what capabilities of Stormbreaker and the Tesseract allow them to access the Bifrost's teleportation.