Pace
I am amazed how in this day and age that a jet of this size can fly for so long undetected! Just shows that if you ever wanted to attack any of these countries they would have no clue about you!
I know that in areas if the world I have flown in Europe you only have to go 1/2 a NM off track and someone jumps down your throat!
Even in Africa you are giving constant position reports and estimates as well as talking too other aircraft so this is amazing !
Why ?
Only one satellite over the Indian Ocean used primarily for ACARs that was switched off, no radar coverage there either, limited primary radar coverage on the original route, you can disappear in Scottish airspace quite easily.
And of course flying in Africa, well you answered your own statement, they use position reports like so many places in the world because of limited radar.