Tristana's E also deals DoT, which means it's never dealing it's true damage and it can't be used as a nuking spell and it's cooldown is rather long, unlike Cass's, unlike Annie's, unlike Kass'. Kass' grants a shield, Annie's refunds most of her mana, Cass' can be used time and time again. Akali's is pretty much free and heals her for a ton of health.
Tristana's is on a large cooldown and reduces healing.
Gragas' ult used to have a huge base damage with insane AP scaling and they only just recently nerfed it, and it was AoE too.
Lucian's ult can have up to 330% AP scaling. with a base damage os 1980 if he lands every shot, which isn't too difficult.
Malzahar's ult has a 130% AP scaling ratio with a 550 base (Tristana's base is only 500), which is almost as powerful as Tristana's R, but he can hold them in place to deal a ton more damage with his W. For that matter, all his spells have an 80% AP scaling except his ult, which is crazy since they all have high base damage as well.
Morgana's ult deals 650 flat and has a 140% AP scaling, and it hits everyone, and then STUNS them, and if she hits a binding she deals 300 flat with 90% scaling and snares you for three seconds, not to mention if you're forced to sit in Tormented Soil, it deals 600 + 165 AP scaling.
In regards to level 1 spells, most champions have more than one that can be used safely to hit someone. Tristana does not. Tristana has her jump which can easily kill her and actually deals less damage than the normal 80, and she has her E with twice the cooldown time of everybody else's spells. Is Riot going to reduce the cooldown on it? I would be OK with that. Is RIot going to not compensate at all? Not OK.
There are champions who are just as strong or stronger than AP Trist in regards to AP scaling ratios. These champions are also generally more useful than Tristana her only job is to act as an AP assassin and eliminate one target before being semi-useless due to her major damage being down and only being able to Auto-attack and jump for a good 10-12 seconds until her E comes back (cooldown reduction brings it down from 16 seconds)