It is because the jews did not actually believe in the Old Testament. They had created their own traditions and doctrines which remain today known as Judaism. Contrary to popular misconception, the followers of Judaism do not believe in the Old Testament nor the Torah, but they follow the heresies that their wicked fathers and they themselves made. For this cause even one person will accuse them to God, this person is Moses.
The whole chapter of John 5 is themed around this disparity in the jews' disbelief in Jesus and what the Old Testament scriptures say, and how they made up their own traditions, but since it is too long to post in full here, here is an excerpt:
John 5:39-47
39 Search the scriptures; for in them ye think ye have eternal life: and they are they which testify of me.
40 And ye will not come to me, that ye might have life.
41 I receive not honour from men.
42 But I know you, that ye have not the love of God in you.
43 I am come in my Father's name, and ye receive me not: if another shall come in his own name, him ye will receive.
44 How can ye believe, which receive honour one of another, and seek not the honour that cometh from God only?
45 Do not think that I will accuse you to the Father: there is one that accuseth you, even Moses, in whom ye trust.
46 For had ye believed Moses, ye would have believed me; for he wrote of me.
47 But if ye believe not his writings, how shall ye believe my words?