Why are not now Muslim countries well developed in science & technology whereas this was not the case in era between 8th-12th centuries after BC?
Because, like some of the Church's stands on science, Islam also had its bad leaders that professed that some sciences are the work of the devil.
There is a very good talk by Neil deGrasse Tyson that explains this.
But even after that, unfortunately, a lot of it had to do with the fact that most Muslim countries were subject to colonial presence, which greatly slowed their potential for development. In fact, most of the Muslim countries were flourishing at the time when Europe was in the dark ages. If you understand French, this is a short but very nice video that speaks to that effect by historian René Vautier.
So to sum up, Muslim countries had their great period of grandeur, then collapsed a little, then had a fertile enough ground (more fertile than in the western world at the time) but did not have the chance to evolve.
But I am reluctant to close my answer without saying that lamenting on the past is one thing, but taking responsibilities for the present is another. In today's world, the biggest problem of Muslim countries, in addition to corruption, low level education, and huge political problems, is religious fundamentalism. But that is very close to the context in which Galileo and Copernicus evolved. So one should not take it as an excuse.
Muslim countries are getting there. Colonialism and western domination only ended less than a century ago, give it time.
In that regard, I completely disagree with the assertion that it has anything to do with there not being enough "good Muslims".
There is also a very good article by the Economist on how Muslim countries are "making a come back".