Why is the Tomb of the Unknown Soldier guarded 24/7 and who initiated the order to guard the tomb 24/7?

There are many such tombs.

The modern trend was started by the United Kingdom in 1920 when it buried an Unknown Warrior on behalf of all First World War British Empire forces in Westminster Abbey. This was a significant gesture since Westminster Abbey is the place where monarchs, the nobility and other people of high esteem are buried. Part of the inscription on the stone reads:

"They buried him among the kings Because he Had done good toward God and Toward His house."


https://en.wikipedia.org/wiki/Tomb_of_the_unknown_soldier

Since April 6, 1948 the Tomb of the Unknown Soldier has been guarded 24 hours a day, 365 days a year, regardless of weather.  Millions of Americans annually visit the Memorial Amphitheater to watch these dedicated soldiers of a new generation, dutifully pace 21 steps across a black mat on the west face of the Tomb.  It is a time honored ritual, executed with great precision and filled with American pride.

http://www.homeofheroes.com/gravesites/unknowns/0_unknowns_sentinels.html

Here's the "why" it is now guarded:

Has anyone ever tried to get past the Tomb guards, or attempted to deface the Tomb?

Yes, that is the reason why we now guard the Tomb. Back in the early 1920's, we didn't have guards and the Tomb looked much different (see attached picture). People often came to the cemetery in those days for picnics during which time some would actually use the Tomb as a picnic area (probably because of the view). Soon after, 1925, they posted a civilian guard; in 1926, a military guard was posted during cemetery hours; and on July 1, 1937, this was expanded to the 24-hour watch. Since then, the ceremony has developed throughout the years to what we have today.

http://www.news-gazette.com/news/local/2012-05-27/facts-about-tomb-unknowns.html

I have seen the US tomb and the USMC guard who walks in front of it. It is a most beautiful sight, and certainly creates a strong feeling of love of country.