I am pretty biased, but here goes nothing.
The reasons I see why people don't play him;
- No CC
- Immobile
- Only deals damage and healing
- Looks terrible
- Can be severly abused
- Binary
- SUPER GIMMICKY
- Team reliant
- Item reliant
The reasons I see why people don't play him:
- Tons of damage
- Delicious value in kit
- Strong early/mid/late
- Scary objective control
- Most badass champ in league no Q's asked
- Clutch Soraka level heals
- Oppressive passive
- Memes
- Pizza feet
Honestly, the biggest reason I could see people NOT playing Morde is because of how gimmicky he is. As a matter of fact, I stopped playing him as much as I used to because of this.
You literally sit in lane, and pray the enemy is stupid enough to duel you early. Level 3 you can kill anyone. ANYONE (only if they are stupid). He is extremely reliant on Rylai's and an ally to cast his W. If he can't use his W, he gets melted in half a second without tank items and deals pitiful damage.
Sure Mordekaiser scales decent into lategame due to his high scalings and delicious value in his kit, but the lack of mobility, CC, and atrocious base stats just makes him a walking statue of gold (see M.M's gif) early game. The only chance you have once you catch the enemy junglers attention is either coordinating with your jungler to countergank and destroy the enemy (rare in low Elo from my experience, I am low Elo) or lose your ability to go super agro.
Max E first? Die from health costs and become weak to engage.
Max W first? Become completely reliant on landing your W heal/damage.
Max Q first? Your other abilities will be too weak.
TLDR; Mordekaiser is very, VERY gimmicky and honestly just a block of cheese. He also has EXTREMELY abusive flaws. It discourages me to play him more and more often. I only reccomend him in Bronze Freelo and Silver if you can play him right.