There is no doubt this truly happened, but in many of the events in the gospels, they are the only records of the history which is why they written. I think when one gospel has something and the others do not, we can assume this is not to be central in our view of the ministry of Christ, but that it is important from the angle that the individual writer takes.**
For example take Alfred Edresheims way of categorizing the gospels (which I like):
Matthew (Jewish view of the Christ) - has for its main object the Discourses or teaching of the Lord, around which the History groups itself. It is intended as a demonstration, primarily addressed to the Jews, and in a form peculiarly suited to them, that Jesus was the Messiah, the Son of the Living God.
Mark (general view of the Christ) - is a rapid survey of the History of the Christ as such. It deals mainly with the Galilean Ministry.
Luke (Gentile view of the Christ) - complements the narratives in the other two Gospels (St. Matthew and St. Mark), and it supplements them by tracing, what is not done otherwise: the Ministry in Peroea. Thus, it also forms a transition to the Fourth Gospel of the Judæan Ministry.
John (the Church’s view of Christ) - which gives the highest, the reflective, view of the Eternal Son as the Word, deals almost exclusively with the Jerusalem Ministry.
With this category of Mathew we could say why did Mathew think the many dead raising to life and the thunder was important along side of the curtain tearing? We must keep in mind the gospels only select certain highlights of the miracles the Lord did and we can assume those that are recorded are only a drop in the bucker, juts as the words he spoke were much, much more than what was recorded. The words in the gospels are just little snippet summaries.
One possible answer is that Mathew, as writing to a Jewish audience, really wanted to stress the end of the Jewish Law and power in Christ’s death to end it. Besides the Jews expected Messiah to bring about a resurrection of the dead soon after his arrival to this earth (see here). The other three may have not have included it in their limited selection of material, because the focus should really be on Christ’s death at this point, who cares in a sense about his miracles now that the great event has arrived. So this is a great miracle and thankfully Mathew recorded it, but we must not let any miracles cloud the greatest event which was Christ’s death.