Many regard this as being the consequence of several factors. Firstly, the Apostles who were the divinely appointed witnesses to the resurrection had all died. Secondly, the canon of the New Testament scriptures was complete, although not organized structurally as it is today. Some believe that the use of miracles was thus no longer a necessary part of the witness to the truth or needed as a confirmation that Jesus was indeed the Messiah. If the miracles also served as a confirmation of who were God's appointed servants, and there is some suggestion that this is so, then this obviously was no longer needed as they passed from the scene.
It also needs to be stated that although the frequency of miracles and the open and seeming public nature of them has indeed declined to a marked degree, this does not mean that such things no longer occur. Although one needs to exercise caution in the light of all kinds of deception, it appears that miracles do occur in various places as God deems fit, and this seems usually to be in similar circumstances to the early church. That is, where the Gospel is being spread and the powers of darkness are being confronted and particularly where those concerned are specifically not 'seeking a sign' God often does wonders to honor His name.