Aside from the fact that Mako had too goddamn much screentime as it was (and still had more focus over Asami even during 3-4,) all the screentime/focused episodes in the world doesn't mean dick if the creator(s) have no idea who the character is.
I'm going to be hitting the Doylism Drum once again (and any/everytime it's applicable,) because Mako is very much "cart before the horse" in design whether it was because Bryke didn't have enough prep time, they suck at characterization unless directly focused on it (ex. "Korra Alone") and need a writer like Ehasz to do the heaving lifting (ex. "Old Wounds" definitely has a more personal feel than what DiMartino would write) or both. If not by their introductory scene then by the end of the episode or end of the season, you, (audience and/or author) are supposed to have a clear idea who the character is and what their deal/trajectory is.
Aang: A fun-loving Reluctant Hero that will have to mature and embrace his destiny.
Katara: A motherly yet take-no-shit young woman that aspires to be the best waterbender possible and deal with the loss of her mother.
Sokka: An immature, sexist wannabe that will grow into a true warrior and honorable man regardless of no bending ability.
Toph: A tough, snarky, independent young woman that refuses to let her disability hold her back or be coddled as she learns that accepting help isn't a sign of weakness.
Zuko: Spiritually/emotionally torn between pleasing his father/following the status quo and doing what his heart knows is right.
Korra: An inverse of Aang that starts out as a bratty goddess that matures/learns to be human, accepting the warts and all in herself and others.
Bolin: Comic relief that'll eventually mature without losing his heart.
Asami: Romantic rival to Korra that seems to good to be true and will unsurprisingly be ev-no wait, she actually IS that sweet, generous and loyal to her friends.
Mako: …*crickets chirping*
As "The Problem with Mako" thread pointed out, even within Book 1 as opposed to what happens after it, there's no consistency at to who he is, what he wants, why he wants it, etc., simply a pretty face that's whatever the episode needs him to be and that's usually a tool to milk as much teen drama out of a scene as much as possible (ex. dragging out breaking up with Asami for the sake of dragging it out even though we're supposed to sympathize with him, somehow).
"Part of a multiracial family representing how times have changed, talented firebender, Zuko without the angst." That blurb was clearly what Bryke had in mind and only what they had in mind, which is exactly why there's no consistency to who he is because he has no greater goal/character arc to anchor/focus him, so there's no real evolution to who he is from his debut to when he's puking out his feelings to Korra at the end.
Instead despite plenty of material to work with given his background (ex. Tunnel-vision toward Amon for kidnapping his brother, his feelings as a firebender who lost his parents to a firebender,) like Mako himself can't be bothered to think twice about Asami after she calls him out on cheating, Bryke can't be bothered to think harder of who he is/could be, settling on "Token Love Interest" as his anchor so regardless of Asami not being evil as originally planned, (which would've been eye-rollingly cliched,) only the conforming endgame of kissing Korra and he/they awkwardly/pointlessly stall until then "because that's how the formula goes" even though it makes him look like a dick, wastes everyone's time, makes everyone look bad, contradicts everything the franchise stands for and could've been avoided when confronted in the kitchen right there on the spot if not never implemented in the season, period. He coasts on being the male lead love interest no different than the bland White Male Lead in a show/movie that claims to be so "diverse" even though/despite being the least interesting and contradicting the show/movie's themes.
So while Book 2 sets out to deconstruct/sink Makorra and Masami, there's still a cart before the horse when it comes to Mako's new occupation.
Why is Mako a cop?
Bryke (shrugs shoulders, mumbling): Dunno, but he looks cool, right?
Again, no clear idea to why he wants to be a cop, let alone detective ("Remembrances" vaguely implies Korra's heroism inspired him to do it,) when something like atoning for his criminal past and/or wanting to make an honest, respectable living could've been motivations, but that never happens, just shilling right down to the couple of slobs designed to make him look good and Asami being chickified to let him hijack her sub-plot. Bryke remembered to make him look/feel guilty for what he did this time, but while Book 3 onwards takes the piss out of him (basically admitted "we fucked up") and have him low-key atone for his past mistakes, he's still the Token Love Interest only one defined by his failures rather than any new goals or insight to the extent he's the only one going into Book 4 with nothing personal on the line compared to even supporting characters like Varrick having specific drives.
It's also partly because of this (and status quo/heteronormity) that Mako stans are so bad for trying to ignore his faults and romanticize him into a better boyfriend than he really was and act like he was "cheated" out of ending with Korra (or at least Asami) and blame Bryke and/or "Social Justice Warriors" for "making" Korrasami happen instead.
TL;DR: No point in Mako having more episodes when the writers have no idea what to do with him and he's so devoid of character that his stans project onto him.