Teleport has more counterplay than any other summoner spell in this game...
Unlike other summoner spells, where the effect is instant, teleport is a channel that can be interrupted EXTREMELY easily (including by yourself accidentally sometimes). You can also just kill the person outright before the channel even finishes. So why does this spell have the longest cooldown?
I'd also like to point out that the (fairly) recent teleport nerf was an indirect nerf to tanks. Why you ask? Tanks have trouble making plays by themselves. They don't do much damage to towers or objectives and they can't 1 v 1 unless extremely fed. Tanks do best when they have an ally with them (preferably someone who has damage). So as you can see it's not just itemization and runes screwing over tanks this season but summoner spells too! Riot is truly doing everything in their power to make sure tanks are useless this season.