I'd like to offer two possible answers:
According to the Ramban, miracles such as the “splitting of the sea” were performed in the past to teach that God is Omnipotent and is always ruling over all of Creation. But, God doesn't perform miracles in every generation just to impress non-believers. Rather, He performed the miracle(s) then, and afterwards He commanded us to remember them via numerous mitzvot, such as mezuza , kriat shema and Pesach. By recalling these “open” miracles a person can also become aware of the “hidden” miracles. A basic tenet of the Torah is the belief that all occurrences are miraculous and are not merely “nature”.
So, in answer to your question, God is in fact “speaking” to us all the time, and it's our job to be attentive!
Another possible answer is the following:
When the Biblical generations experienced miracles, they were impressed and showed their recognition of God's hand by living according to God's word. The miracles taught a lesson to a People prepared to learn. They possessed a pure and wholesome faith.
Today, however, if miracles were to occur, we would explain them away scientifically. We would lose sight of the miracle and be satisfied with some natural explanation. So, what effect would miracles have today? Miracles are a means of communication, and communication requires two sides. When we are ready for miracles, when we can recognize one when it hits us, we will have them.
Our Sages teach us to “Praise Him for each breath we draw.” We are supposed to be grateful for each heart-beat, for all of our vital functions, and, for that matter, all of the day-to-day workings of nature. This teaches us that there are miracles all around us, even if they occur seventy times a minute. “But it's perfectly natural” is the usual rhetoric. Yet, this is exactly the point. The ordinary, the natural and the commonplace are as much the works of His hands as the splitting the sea and Creation itself.
Science seeks the natural in the supernatural; whereas the Torah shows the supernatural in the natural.
