Means “Precisely because”. The clause following usually expresses regret or some other unhappy emotion.

need to know this for JLPT N2 grammar

田中選手が今シーズン活躍するのを楽しみに待っていただけに彼の怪我のニュースを見て驚いた。
As I was really looking forward to seeing Tanaka actively playing this season, I was surprised when I saw on the news that he got injured.