Since the Old Testament in many places talks about the "everlasting covenant" with the Israelites, why is it no longer everlasting?
The word "everlasting" does not necessarily mean "infinite." The world is described as lasting forever, but we know from II Peter 3 that there is a terminating point. Instead, the correct definition is "a long, but indeterminate amount of time."
The word comes from the Hebrew word "olam." It is used to refer to the ancient past as in Deuteronomy 32:7, yet the past is not of infinite length -- it had a distinct beginning. The word can even be used in reference to one's own life time, as in Psalms 77:5 or Exodus 21:6. Perhaps I will have a long life, but the length of my life is indeterminate to me -- I do not see its end, yet I know that it will certainly come to an end.
Hence, the Old Covenant can be accurately described as "everlasting." It's reign was for a long, but indeterminate period of time. The use of the term does not rule out an ending, only that the ending was indeterminable to mankind.