Why does Attack on Titan's Armin hardly ever transform into the Colossal Titan? As the very first "shifter" to make their presence felt in Attack on Titan, the Colossal Titan holds a special place in Hajime Isayama's mythology. When the story begins, the Colossal Titan is possessed by Bertholdt Hoover - one of three undercover soldiers sent by Marley to steal the Founding Titan. Bertholdt retains that power until Attack on Titan season 3, when he's outsmarted and defeated by a self-sacrificing Armin Arlert. Thanks to the efforts of Eren and Mikasa, Armin is saved by eating Bertholdt and inheriting the Colossal Titan for himself.
Since then, we know Armin wielded the Colossal Titan to protect Paradis Island from Marley's ships during Attack on Titan's pre-season 4 time skip. Armin also utilized the Colossal Titan's destructive prowess to decimate a Marleyan port during Attack on Titan season 4's assault on Liberio, detonating the surrounding area, then trampling whatever remained under those ginormous feet. Weirdly, however, Armin hasn't transformed since. Most Titan-shifters change form for every battle (Eren's Attack Titan, Reiner's Armored Titan, Annie's Female Titan, etc), but Armin has typically preferred to don his 3-D maneuver gear and join the regular soldiers instead.
Notable instances of Armin forgetting about his Colossal Titan powers include Attack on Titan season 4's Shiganshina battle against the invaders from Marley, and the port clash against Floch's Jaegerists (for which Reiner and Annie both transform straight away). One potential explanation is the impracticality of possessing such a massive, cumbersome Titan compared to the other shifters. You could absolutely argue that the Colossal Titan is unsuitable for most situations - especially the port battle, where Armin can't risk destroying the Azumabito clan's "flying boat," or hurting nearby allies with the Colossal's burning steam. But that doesn't explain Armin not Titanizing during the Shiganshina fight, where the battlefield was much larger. Also, Bertholdt circumvented the Colossal's size problem by using partial transformations (the head and shoulders atop Wall Rose in Attack on Titan season 2, for example).
Another factor is Armin's unrivaled brainpower. Levi didn't revive Armin over Erwin because he thought the young soldier was a Titan-slaying machine like Mikasa. Armin is valued squarely for his intelligence and strategic brilliance more than his brawn - even if said brawn packs the power of a nuclear explosion - and becoming the Colossal Titan would make relaying instructions and reading the battlefield significantly harder. As we saw in Attack on Titan season 4, however, Armin can eject from his Colossal Titan body, so it's not like he couldn't fulfill both purposes.
While the practicality of becoming a 50-meter, burning hot monster, and Armin's value as a military strategist may half-explain his reluctance to use the Colossal Titan, that's not the entire picture. Ever since Armin took the Colossal from Berholdt, immense guilt has been weighing young Arlert down. Numerous times throughout Attack on Titan season 4, Armin solemnly admits Erwin should've been saved instead, questioning whether he actually deserved the second chance his Colossal Titan provided. Through the Paths, Armin has also started empathizing closely with Bertholdt, which makes wiping out Marleyans morally tricky. It's no secret that Armin feels terrible about destroying Liberio's port, and it's a feeling he'd rather not experience again.
Armin's role as a tactician and his Titan form's ungodly size surely influence Armin's relative lack of transformation scenes compared to other shifters. But the guilt of how he became the Colossal Titan, added to the remorse for how many deaths his transformation has caused thus far, is perhaps the overarching reason Armin uses his monster form so sparsely in Attack on Titan.
