The biggest problem with Du is that the TRA has set it so that Etisalat and Du do not actually compete on price for local calls. i.e. one cannot undercut the other.
So basically you have a new phone company that has no advantage:
(1) Their network is poorer with less coverage.
(2) Their billing systems are terrible.
(3) They charge the same, by the second, but I think Etisalat has that option now too, so no advantage.
(4) You get an 055 number which means you're constantly correcting people... "no it's OH FIVE FIVE!"
The upshot is unless they come out with an amazing service or do something amazing with the price, they will always be a smaller player.
As for their supposed high number of customers... well, a clear majority of those are those of us forced onto Du for Internet and Pay TV as we cannot get Etisalat.