Hi, resident Pantheon main for toplane (though I'm an unranked scrub who only plays normals so I can't say I'm an expert) and I'd like to offer my opinion.
First let's take a look at champions you are most likely going to meet in toplane this season:
. I'm basing this off my own experience, so this may or may not be true.
: This guy basically outlasts you in a straight up fight. With his slow and hook combined with his Q, he can basically come out with more overall health than you. He's a much more painful lane bully, especially when he plays smart (like forcing you under his tower with his E). At level 6? Good luck dealing with him.
: He's not as bad as Darius, but he counters you pretty hard with his Silence (even if you block the AA with your passive, you'll be silenced) and his passive heal. Now you can stop the heal if you make sure to poke him at the right times, but he doesn't have to walk too far away from his turret to avoid the poke. His level 6 problem used to be the same as Darius if you killed him a few times, nowadays it's unlikely you'll be the first villain, but at low health it won't matter.
: Pantheon's passive is her counter. I cannot say I've ever had a frustrating match against a Fiora, she's fun to play against with Pantheon. That being said, she has a strength that all of these other champs have that I will get to.
: Oranges and Barrels. With his ult range he can keep toplane occupied and offer help to other lanes. I may have just performed poorly, but I could not take down his tower. He couldn't take mine either.
: Everyone seems to hate fighting her. A good shield bash to the face can stop all her jumping if timed properly.
: I haven't seen him awhile. I just hate fighting him.
Now what do these champs have that Pantheon lacks? A good mid to late game if they aren't feeding during lane. 
(and 
) are very powerful early game champions. They are meant to be lane bullies in top lane (I know Pantheon can jungle, I'm arguing top), to shut down their opponent. But when you have champions that can outlast you in top lane after getting enough CS, then you need to work twice as hard with an old (fairly useful but a tad unsatisfying at times) kit that has a few annoyances. Plus you have to consider that Pantheon (despite what some people seem to think) is not a tank. He's a fighter/assassin Hybrid, like 
and 
, these are champs that are known for being relatively squishy ('s ult leads away from this thought, but it's really not hard to bring him down to that low) and not having good late games unless fed. Pantheon does decent if he's fed, I've carried a few games with him actually, but that's only because I could shut down the opponent multiple times (and I had a lot of close calls in lane). In teamfights he has a 1 second CC that brings him into the fight, a cone attack (which is pretty powerful against champions, more so than minions) and (perhaps the only thing he can offer if he's not doing well) a clean up ability (AKA: His E's passive combined with his Q).
As for why he isn't played often in top? Well to be perfectly honest, early game top doesn't seem to matter in the long run. I've often wondered what top is suppose to be for. Midlane is the shortest route and easiest to gank (maybe) so usually you want to push that lane hard. Botlane has the ADC (usually if not always a marksmen) and support, and since the ADC seems to be the most important role for winning (cause of tower kill potential) they must get fed or shut down early. I've seen games where 4 of the enemy team rush bot to shut down the ADC.
But top? What's important about top? You mostly have your guys like 
, etc. Champions who are melee, considerably tough, and can dish out damage. They just seem to duke it out to get stronger so they'll be able to provide a frontline for teamfights. If you're getting bullied by the guy chucking a spear, then you can sacrifice your tower while starving him of more kills while you CS under the second and leave him to be attacked from behind from over extending (which he'll do if he doesn't just roam after the first tower. Which I do a lot because having two gankers helps). If all you need is to farm CS to catch up, then you're good. If you offer more utility/peel than straight up killing power in teamfights you're good. Or you could just hold him off so he won't gank because he doesn't want to lose his tower.
(Side note: Yes I know assassins are played in toplane. I tend to see the jungle as the assassins playground).
So why isn't he played toplane? Because** toplane doesn't matter early game.** He's not a great choice for top (but I still love playing him there) and makes a better jungler (I suck at jungle and there are better junglers. I want to say it's personal preference, but there are better junglers that also scale into late game better). So unless you're on a team that pushes the finish the game early and works together, a strong early game lane bully that falls off mid to late is not a great toplaner when you could have a tank instead.
He's fun, underrated, but requires more effort ("oh but he has point and click abilities" they also cost a lot of mana. He has no escapes, so it's usually do or die with him. If he's running away from a fight he can't win, he's most likely going to die in his attempt if he doesn't have 
up. Especially since his only CC sends him directly to the guy chasing him and lasts 1 second) to be successful in the long run and I suppose people don't want to deal with that. People may also find him boring to play, I really only like his ult for it's positioning, I find his W boring, and his E active dull. He's not rewarding unless you put in a lot of effort with success, he's not dangerous late game unless you're at low health (and with all the escapes people have, you can probably get away), and one of his best strengths is in a lane that doesn't matter. You could say he's great at split pushing, but that's only because he can get to a tower faster. He doesn't hit fast enough or hard enough without items. is a better pusher.
It's more a result of the meta than anything else. I'm not gonna stop maining him in toplane, I find each victory I get with him very satisfying because of how unchanged his kit is compared to the rest. But he has a much harder time coming back if he's shut down, sometimes he doesn't.
Just my 2 cents.
