Why can't it be eternally existent?

A Beginning or End Date. We have found the Universe's "age" to be 13.82 billion years old. In addition to this, There is the estimated end of the Universe; Known as the "Heat Death of the Universe," In which the universe has diminished to a state with no thermodynamic free energy and therefore can no longer sustain processes that consume energy (including computation and life). Heat death does not imply any particular absolute temperature; it only requires that temperature differences or other processes may no longer be exploited to perform work. These are two massive things preventing the Universe from being eternal, because Eternal Means: lasting or existing forever; without end or beginning. Only One Half of these two things have to be correct for this to mean the Universe is not eternal.

Read more about the ESA's Planck Mission "The image is based on the initial 15.5 months of data from Planck and is the mission’s first all-sky picture of the oldest light in our Universe, imprinted on the sky when it was just 380 000 years old.
At that time, the young Universe was filled with a hot dense soup of interacting protons, electrons and photons at about 2700ºC. When the protons and electrons joined to form hydrogen atoms, the light was set free. As the Universe has expanded, this light today has been stretched out to microwave wavelengths, equivalent to a temperature of just 2.7 degrees above absolute zero."

"A few years ago, the WMAP spacecraft looked at the Universe much as Planck has, and for the time got the best determination of the cosmic age: 13.73 +/- 0.12 billion years old.
Planck has found that the Universe is nearly 100 million years older than that: 13.82 billion years."