Probably to mirror the electoral college in the general election, so that a candidate can sort of get a feel for the general election process.
So why use the electoral college? Why can't the people decide by majority rule? Originally people didn't have cable news so the founders thought as a failsafe (they thought the people might elect someone unqualified) they'd create the electoral college, which doesn't work anyway because the winning candidate gets to choose whose in it.
Finally to answer your original question, tradition, and the fact that not enough people have spoken up for an amendment to change it.
presidential primary elections and caucuses
The territories have no vote in Presidential elections. They do send voting delegates to the national nominating conventions.
delegates from each state select political parties'nominees for president
This refers to the system where the results of primary elections direct delegates to vote at the national convention to select the candidate for the presidential election.
National convention (A+)
The presidential elections are held every four years.
Delegates from all the states meet and elect a candidate for president and then for vice president, Nowadays so many of the delegates are committed to a particular candidate by primary elections that one candidate can have the nomination "sewed up" before the convention occurs and lately it has been the custom to let the presidential nominee pick his running mate. This did not used to be the case.
Presidential Elections
Elections in the USA occur every four years. There where presidential elections in 1992 and then in 1996.
elections, '80s
The presidential suite was a big big.These presidential elections are so boring.
Delegates are chosen by primary elections in which anybody can vote, as well as by party caucuses, that is by election by party leaders.