Plenty of people realize Donphan's worth. I'm sorry to say it, but it is completely inferior as both a deck and as a tech compared to Terrakion and is currently obsolete.
Allow me to list the attackers more commonly found in higher tier decks in the current format.
Tornadus, Thundurus, Zekrom, Zekrom EX, Terrakion, Groudon EX, Darkrai EX, Zoroark, Tornadus EX, Mewtwo EX. Am I missing any?
Against Tornadus, Terrakion prize trades without needing any extra cards if the Tornadus doesn't have Eviolite, and if Tornadus does have Eviolite Terrakion only needs a single PlusPower or RoAto prize trade. Donphan on the other hand needs a Rocky Helmet, RoA, or 3 PlusPowers to prize trade if Tornadus doesn't even have an Eviolite. With an Eviolite, Donphan needs a RoA AND Rocky Helmet, or one of them and 3 PlusPower, all of which are unsearchable.
Against Thundurus both Terrakion and Donphan win the prize trade so nobody is dropping it. Nothing much to say there.
Against Zekrom, Terrakion wins the prize trade unless the Zekrom uses a PlusPower. This does happen, but it makes Zekrom's KOs less consistent than Terrakion's and Zekrom has no way of actually winning the prize trade so Terrakion will eventually win. Against Donphan, Zekrom does indeed need 4 PlusPowers (note that its only 2 however if your own RoA is in play) but the major difference is that if Zekrom has an Eviolite then Donphan can no longer OHKO it, leading to even prize trades.
Against Zekrom EX Terrakion has a hugely advantageous prize by OHKOing for 2 prizes while getting OHKOed back for 1, but Zekrom EX can use an Eviolite to turn that into an even prize trade. Even so, with just a PlusPower Terrakion turns that into an advantageous matchup again. Donphan also has an advantageous prize trade, but with a single PlusPower or single turn Earthquake damage Zekrom EX trades prizes, but is easier to keep rolling since its a basic KOing two Stage 1s.
Terrakion IS Terrakion, obviously even prize trade there no matter what. However, Donphan loses the prize trade to Terrakion unless it has a PlusPower or Rocky Helmet, in which case the matchup is still even and Donphan has no way of making the matchup advantageous.
Groudon EX and Terrakion 2HKO each other but due to the two prize rule Terrakion comes out on top unless Groudon has an Eviolite (very unlikely since its usually run with Exp. Share in Terrakion) in which case Terrakion can still win the prize trade by taking 2 prizes in 3 turns while Groudon takes 2 in 4 turns. Donphan does beat Groudon EX in the prize trade as well, taking 2 prizes in 3 turns while Groudon takes 2 in 4 turns...unless Donphan has two or more damage counters previously from Earthquake recoil and Tromp damage, which is very possible, in which case Groudon ties or wins the prize trade.
Darkrai EX gets OHKOed by Terrakion while it 2HKOs back, and its an EX-obviously advantageous for Terrakion. However, with the bench damage, Darkrai can pull off an even prize trade by utilizing a Catcher and Special Dark, Darkness Claw, or 2 PlusPowers. Against Donphan Darkrai also loses the prize trade, but not quite as steeply, giving it more time and the fact that nearly all Darkrai run Tornadus (EX) anyway means it will remain close enough to catch up and win.
Zoroark gets OHKOed by both Terrakion and Donphan while needing more cards to OHKO them back, so while it can maintain an even prize trade it generally loses against both of them. However, Weavile, a card often run in Zoroark decks, is more crippling on Donphan than Terrakion because Exp. Share, energy, and Terrakion can all be played down in one turn while Donphan remains in the hand for a turn where it can get discarded, making swarming more difficult.
Against Tornadus EX Terrakion wins the prize trade without any extra cards so long as Tornadus EX doesn't have Eviolite, and even with Eviolite its an even prize trade turned once again in Terrakion's favor with RoA should they run it. Donphan meanwhile 5HKOs Tornadus EX while getting 2HKOed back, and awful prize trade. With Rocky Helmet or RoA it can win the prize trade, but that's still less consistent than Terrakion by far. In addition, should Tornadus EX have an Eviolite, it will be 9HKOed-9HKOed! by Donphan while still netting the 2HKO back and Donphan needs both RoA AND Rocky Helmet to return to an even prize trade.
Against Mewtwo EX Terrakion gets 2HKOs while getting 2HKOed back (unless Mewtwo can attach 6 energy or 5 energy and a PlusPower, either way unlikely) and even with an Eviolite Mewtwo still has an bad prize trade. Donphan gets 2HKOed as well, but it only 3HKOs back so the prize trade advantage is not as good. Also, when Mewtwo uses Eviolite against Donphan, Donphan loses the prize trade unless it gets a PlusPower or Rocky Helmet, while it needs Rocky Helmet AND PlusPower to win the prize trade still.
So based on the fact that every main attacker in the format has a better prize trade against Donphan than Terrakion, why do you think Donphan is better?