The popularity lies in the poem's egalitarianism and  in its reminder "Memento Mori" (Remember you will die). 

In the end, possessions and fame will mean nothing. "Far from the madding crowd" we are all alike in death.  Death is the great equalizer: 

Can storied urn or animated bust
Back to its mansion call the fleeting breath?
Can Honour's voice provoke the silent dust,
Or Flattery soothe the dull cold ear of Death?

Another factor contributing to the ongoing popularity of this poem is the acknowledgment that some who die have never been given a fair chance in life.  As the speaker thinks about who might lie beneath those "many a mouldering heap" (freshly dug graves) he wonders what these simple peasants might have been had they means and opportunity to accomplish more in their lives:

Perhaps in this neglected spot is laid
Some heart once pregnant with celestial fire;
Hands, that the rod of empire might have sway'd,
Or waked to ecstasy the living lyre:

But Knowledge to their eyes her ample page,
Rich with the spoils of time, did ne'er unroll;
Chill Penury repress'd their noble rage,
And froze the genial current of the soul.