The biggest reason for this is that no Pokemon exists within a vacuum. That's to say, Linoone's tiering isn't based solely on its own objective merits and demerits, but rather how well it fares subjectively with the rest of the meta.
And this applies to practically every single Pokemon in the meta-game. It's why some Ultra Beasts got banned to Ubers, while others struggled to be relevant in OU. It's why Aegislash was in Ubers till Urshifu came along and and made King's Shield less viable. Or why Tyranitar briefly fell to UU.
Likewise, Linoone lingers around whatever tier can reliably check its movesets. Tiers like PU filled with Pokemon with dismal bulk and overall low base stat total wouldn't be able to handle a Pokemon like Linoone without running very specific counters and sets tailored for Linoone - something that competitive players work very hard to prevent. It's also why Cinderace and Magearna got banned to Ubers recently, because team building becomes an issue of using such a strong Pokemon, and then another in place purely to counter said Pokemon. That eliminates 33% of the team diversity right there.
Since tiering isn't an exact science, it relies on a lot of data generated by thousands of players trying out multiple strategies, which can fluctuate as new, previously unthought of combinations (niche movesets, EV Spreads designed specifically to counter meta sets) and ways to beat a meta set emerge.
Linoone doesn't have a lot of bulk to withstand strong STAB moves, nor does it deal enough damage with Belly Drum + Extremespeed to Rock/Steel resists. Hence, it can't ever become a standard meta threat in OU and Ubers, but it does fairly well in tiers where its own checks and counters are absent. In UU for example, Cobalion, one of the staples, resists Extremespeed and can easily outspeed and OHKO with Close Combat, making it less viable than RU or RUBL where Cobalion (just one example) can't threaten it so.
tl;dr: Tiering isn't objective with respect to a Pokemon's stats. Depending on new Pokemon, moves, etc. emerging with newer games, the relative potency of a set is also reflected with an appropriate tier shift.
