This is a good question. One of the reasons why literature from the past endures, like the epic Beowulf, is due to themes that apply to many people from all generations. In a word, there is something universal.
When we come to Beowulf, there are many universal themes that we can mention. I will only mention two of them.
First, no matter what you think about the hero, Beowulf, one of the characteristics that he possess is bravery and courage. For example, he is unafraid to fight Grendel, his mother, or even the dragon. He will trod where no man wants to go. There is something noble about this and this bravery resonates with many people. From a sociological point of view, I do not think there was ever a society that did not prize valor.
Second, there is a sense of good verses evil. The idea that there are two sides in a "cosmic" struggle is another theme that we find in many cultures. It is a simple binary, but it is very powerful.