From my observation, there are two most important factors, and none of them has to with doctrine.
First and foremost, is the shrinking "recruitment pool". The JW's have been historically inept to make significant progress in Islamic, Buddhist and Hinduist countries, so they have directed their efforts mostly into areas where christianity in general has been dominant: Europe, North America, South America, Africa, Australia. (Japan is a bit of an exception, but the JWs took advantage of the collapse of the Emperor's worshipping following WW2).
In the most well-educated and affluent societies of Europe, North America, Australia, access to information, critical thinking skills, more liberal attitude towards sexual orientation, have steadily contributed to a progressive secularization of society, and consequently, less interest in religion. In these areas, the JWs decline is only mitigated by the baptism of born-ins and the recruitment amongst immigrant communities.
The only source of significant growth is South America and Africa, where the erosion of the authority of the Catholic Church and mainstream protestantism, coupled with low rate of high education level is giving the JWs some leeway and helps to make-up the worldwide numbers.
The other factor is demographics. In general, the average JW is aging, the retention of born-ins is alarmingly low, and we should see an acceleration in the decline curve, as the numbers of dying old folk cannot be replaced by the recruitment of new ones and the born-ins aren't enough to make up for the ones who pass away or get DF'd.
I would argue that the most important effect of the internet and the "apostate activism" is not seen in the numbers of those who leave the Organization after many years of baptism, but rather, in the number of born-ins who refuse to become baptized JWs or those who baptized at a young age but who leave the JWs as soon as they get a measure of autonomy. That's why there has been a significant change from the Organization towards baptism of pre-teen youngsters and a tacit incentive to marrying young "in the truth". Catch them before they learn to think for themselves and while their fanatic parents still have enough authority over them and tie them in with all sorts of commitments so that leaving is not seen as a viable option.