As per Mahabharat Tatparya Nirnaya of Madhavacharya, it was because Balarama had come under the influence of kali and had deviated from bhagavata dharam. The reason he deviated from dharma was because he disbelieved Krishna's words, who was God Himself.
Krishna and Balarama chased the fleeting Satadhanwa as he had killed Satyabhama's father Satrajit for the Syamantaka jewel. But after Krishna killed him, He didn't find the precious jewel with him. He told Balarama that the jewel was not with Satadhanwa. But Balarama didn't believe Krishna. He thought that Krishna had kept the jewel and lying to him.
Balarama did not believe Shri Krishna. Out of anger went to Videha and
lived there for five years. Duryodhana, realizing that it is
impossible to get Shri Krishna leave the Pandavas and come to his
side, decided to attract Balarama to his side, and went there. [MBTN -
Duryodhana became the pupil of Balarama for learning the art of mace
warfare. Knowing that Shri Krushna was not around, he requested
Balarama to give his sister Subhadra in marriage to him and Balarama
promised him to grant his wish. [MBTN - 20.34]
If you read the notes that Shri Madhavacharya gives to the verses, it will become apparent how Duryodhana cheated Balarama for his purpose and how Balarama having disbelieved Krishna had deviated from the path of dharma. But Balarama being simple (or probably due to the influence of shani as Acharyaji mentions) was affectionate for his favorite disciple. Probably that is why he favored Duryodhana and ignored his misdeeds like people often ignore misdeeds of persons they like.