than previously thought = than was previously thought = than it was previously thought that it would be.

这就是比较分句中的省略。than通常被分析为连词，than分句的主语为形式主语it+外置主语从句时，可以省略it 和外置主语。当分句谓语为被动语态或系表结构时，某些动词（如thought，planned，expected，believed等）或形容词（如necessary，usual 等），谓语部分可以省略动词be，只保留过去分词或形容词。本句中比较分句就是省略后只剩下 than + 过去分词。