Can someone explain to be why Morde isn't viable in a solo lane?

Hmm thats tough because with good teamwork Solo Morde is totally viable. If you want me to list why it doesn't do so well i can do that.

#Iron Man

• in a Solo lane, its ridiculously hard to keep the shield up. You are constantly spamming spells to keep it up because it now decays immediately. This forces the lane to push and for you to use abilities to keep your shield up. Despite popular belief Morde is very squishy without items so poke hits him hard + his spells cost health.
• It actually doesn't offer all that much protection now that it decays immediately.

#Mace of Spades

• All of his damage was pushed into this ability, which forces him into a very binary playstyle "Hit them with the mace"
• Thanks to his whopping 325 MS almost every champion in the game can just walk away from him no problem after taking 1 or 2 hits.
• If your opponent knows what your Q does they know not to go near you when its on its third hit, once its down they will go in on you. All your damage is in the Q so you are pretty much boned.
• His attack speed is horrendous

#Harvester of Sorrow

• Is pretty much useless in a Solo lane as it offers nothing but a mediocre heal.
• Heal barely covers the cost of the ability, unless you hit an enemy champion. Heal is barely noticeable unless you hit 2 enemy champions.
• Severely reduces his waveclear as it deals very little damage.
• His only form of sustain on a champion that uses health as a resource and yet it has an extremely long Cool Down.
• Movement Speed bonus doesn't work, something im sure his MS was nerfed for...
• Did i already mention how useless it is in a Solo lane?

#Siphon of Destruction

• Other then a damage reduction this skill wasn't changed much.
• The extra Iron Man shield it offers does not cover for its lack of damage thanks to the fact the shield will probably be decayed completely in about 1 second...
• His only form of poke but it pushes the wave leaving a champion with 325 MS and no escape or gap closer very vulnerable to ganks.
• If you don't use it to poke you will do no damage and take a ton of poke damage in return... so either get ganked and killed or get poked to death...

#Children of the Grave

• Can still be countered by a single health pot...
• With no way to hit your Q by yourself and your E not doing much damage, the chances of popping it by yourself are low.
• Morde was nerfed for the Ghost Dragon mechanic, something you probably wont use as a Solo Laner who is probably losing lane.
• Its damage is crap...

Basically Morde is too squishy for a Solo laner, he relies on hitting Q but has no way to do it by himself other than his enemies lack of skill. Hitting 6 does not give him a power spike, if you are dying with ease before level 6 you are going to continue doing so after level 6. He is heavily reliant on teamwork, which is fine with a great team, but most of the time teamwork just isn't going to happen. As a Top laner you are very far from Dragon, forcing you to teleport which limits your use to Bot lane or Mid lane and leaves your lane wide open.

Sorry for the long post, thought id give you a full rundown.

Depends on what you mean by viable.
Viable = can pick him in champselect. And walk to lane? Viable= have a contest of skill for lane control?

Solo Morde outcome in lane is determined at champ select. Barring a noob walking into Morde and fighting in melee range. ( this is why they think he's OP) You can't sustain a shield long enough to trade effectively. Especially considering the 325 MS. The consistent low winrate regardless of games played shows its not the player learning curve holding Morde back.

Edit: I hope this is helpful

I wouldn't say he's not viable, just not that you're working against your team by doing so. It's more of an opportunity cost.

Mord can go top and do.. relatively well, but by going top you're not taking advantage of W's passive, which actually detriments your team by making less XP be shared through everyone. You're also unable to capitalize on your dragon as safely; it's unlikely that, if your team manages an early dragon, the opposing top laner will let you teleport there to get its ghost.

There's also the loss of W's sustained damage, which actively hurts yourself for not having an ally nearby. It's actually a fairly noticeable drop in damage.

On the other hand, if your jungler is generous, he'll start top so you can kill the first camp for an early level 2 advantage. This is actually pretty noticeable as well; you'll be slightly weaker due to not having that damage in lane, but having the level advantage and a second ability anyway lets you bully some lanes fairly hard.