Why is BJJ more effective than other martial arts?

Jiu jitsu has proven itself in actual 1-on-1 combat situations:  (1) challenge matches, (2) the Ultimate Fighting Championship (Mixed Martial Arts), and (3) the military/law enforcement

1. One of the primary methods of advancing the sport during its early development was to issue or accept challenge matches to test the art against other martial artists, fighters, and/or tough guys.  Jiu Jitsu practitioners consistently won those confrontations, and losses or weaknesses that were exposed resulted in adjustments to the sport (less useful moves/positions were changed or eliminated and more effective techniques added). 
2. Inspired by the challenge matches and subsequent videos, the Ultimate Fighting Championship (UFC) was created in 1993 by Rorion Gracie, Art Davie, and John Milius to showcase the effectiveness of Brazilian Jiu Jitsu.  Royce Gracie (Rorion’s younger brother) entered the first four UFCs.  Despite being the lightest competitor in all 4 events, he won 3 of them (UFC 1, UFC 2, & UFC 4).  He withdrew from the finals of UFC 3 due to dehydration.  He didn’t lose a match and won 11 consecutive victories by submission, a record that still stands today.  Perhaps the only more meaningful attribution to jiu jitsu is that every Mixed Martial Artist and nearly all serious martial artists now incorporate Brazilian jiu jitsu as a core part of their training program.
3. The US Army’s Modern Army Combatives Program (MACP) is based largely on Brazilian jiu jitsu, and its founder, Matt Larsen, is a black belt under Jacare Cavalcanti and a member of the Alliance family of jiu jitsu academies (which includes Lucas Lepri Brazilian Jiu Jitsu & Fitness).