Imu and Roger must have faced each other at some point. Did Imu inflict the disease upon Roger? Was Roger too weak to fight Imu because of his illness?
And, roger being weak is not the reason why he didn't fight IMU.
Mihawk at MF said Luffy has most fearsome ability - to make allies...
Roger wasn't only sick but he also didn't have allies to counter WG might. Another thing is him realising after learning true history that he isn't the chosen one (We learnt in wano that someone after 20 years will come to free them).
So, high possibility that Roger came to realisation that he isn't the chosen one and thus, ushered great pirate era.
And, this brings me to another point...
ROGER NEVER FOUND ONE Piece. HE LIED TO USHER PIRATE ERA. ONE PIECE WILL BE FOUND AFTER WG Defeat