Why does the Pope carry the title "Pontifex Maximus"?

In the Roman Republic, the Pontifex Maximus was the head of the Roman religion. He was the most important of the Pontifices (plural of Pontifex), which were positions in the main sacred college (Collegium Pontificum), which was directed by the Pontifex Maximus. Other members of this priesthood included the Rex Sacrorum (king of the sacred rites), the Flaminii, and the Vestales. The number of Pontifices, elected by cooptatio, was originally six, but this number increased to fifteen in the 1st century BC. The Pontifices served for life. The office came into its own with the abolition of the monarchy, when the sacral powers previously vested in the king were transferred either to the Pontifex Maximus or to the Rex Sacrorum. Today it may still be said that the head of the Roman religion is the Pontifex Maximus, since that is one of the titles claimed by the Pope.
The Pontifex was not simply a priest. He had both political and religious authority. It is not clear which of the two came first or had the most importance. ...

Tertullian first applied the term to Pope Callixtus I, although Pope Gregory I was the first to employ it in any formal sense. Pontifex was apparently a word in common currency in early Christianity to denote a bishop. The office was relinquished by the Emperor Gratianus in 382, and was assumed by the Christian Bishops of Rome. It thus became one of the titles of the Popes of the Roman Catholic Church who hold it to this day. This is unusual in that most of the technical terms of Roman paganism were avoided in the vocabulary of Christian Latin in favour of neologisms or Greek words.

http://www.biblicalcatholic.com/apologetics/a104.htm

The word pontifex is sometimes explained as 'bridge builder', but is in fact related to the Etruscan word pont, 'road', and means something like 'preparer of the road'. The pope still calls himself pontifex maximus.

https://en.wikipedia.org/wiki/Pontifex_Maximus#Etymology

In simplest terms, "Pontifex Maximus" means "head honcho."
http://www.thefreedictionary.com/head+honcho

"Pontifex Maximus (Latin for Supreme Pontiff)."
It means he's the greatest, or chief, priest. The distant etymology of the word doesn't explain why the Pope is so called.

Pontifex Maximus – noun, Roman Religion.
a member of the Pontifical College, which was presided over by a chief priest
Origin:
1570–80; < L: appar. lit., path-maker, equiv. to ponti- (s. of p?ns) bridge, prob. orig., path (see pons ) + -fec- (comb. form of facere to make) + -s nom. sing. ending; the literal application is unclear
http://dictionary.reference.com/browse/pontifex%20maximus

pontiff – noun
1. any pontifex.
2. any high or chief priest.
3. Ecclesiastical.
a. a bishop.
b. the Roman Catholic pope, the Bishop of Rome.
Origin:
1600–10; earlier pontife < F, short for L pontifex pontifex
http://dictionary.reference.com/browse/pontiff
http://en.wiktionary.org/wiki/Maximus