Possibly because it was unknown to the West when the lion was, in antiquity, give that title in the West. *** Another possibility: because the male lion has a sort of "harem" and "extended family" (the "pride"), over which the lion "rules" until he is overthrown. I.e. lions live in a community, which the alpha male lion "rules"...and there is even territory involved (each pride competing with neighboring prides for their territory). The adult tiger, on the other hand, is a solitary hunter.

Lions were bestowed that moniker thousands of years ago & it never was changed. I always found it strange because they didn't live in the jungle to begin with.

Tigers actually live in jungles too. Lions do not. 11/27/23

There's a rabbit hole to go down into here... Let's start with the obvious part to most native English speakers. The lion is referred to as the "king of the jungle," at least as a common idiom. But, when we think of "jungle," we think of rainforests, and the vast majority of lions live in the savannahs or in tropical forests that probably don't quite qualify as rainforests. So, why not the tiger? Tigers can grow bigger, tigers can capture larger prey, tigers are more likely than lions to be encountered in the rainforest, tigers are more likely to hunt people for food, etc. I think that, to properly understand the title, one should look at the history of the language. The title "king of the beasts" probably predates the English language. Lions were originally common in Eurasia, where early people made the first western civilizations. The term "jungle" was not known at the time. As travelers from Great Britain went to India for trade, they began to take interest in the culture there - even hunting. This is where the term "jungle" entered the English language. "Jangala" is the Sanskrit word for "a deserted place" or a "dry uninhabitable place," which is synonymous with the English term "desert." But something was lost in translation, and instead it came to be understood as a wet, heavily thicketed place. Likewise, the Sanskrit word for lion was "simha," which sounded like the word for royalty "singh." So the "simha" of the "jangala" or lion of the savannah, was very likely misinterpreted as the "singha" or royalty of the jungle or rainforest, which was similar to the existing phrase common to Europeans that "the lion is the king of the beasts." Thus the phrase used in Sanskrit storytelling morphed into a "false friend" with the English language to become a new idiom with strong ties to older expressions known to the English speakers of the previous generation. No tigers were involved in that process