why don’t we systematically do proper surveys etc)
There have been scientific studies, of which this is the best:
https://www.pewresearch.org/fact-tank/2018/08/08/why-americas-nones-dont-identify-with-a-religion/
Basically, the top reasons were:
Question religious teachings
Disagree with social/ political issues
Dislike of religious organizations
Don’t believe in God
Find religion irrelevant
Dislike religious leaders
Other studies I have seen that ask different questions generally put conflict with religion with science, and social/political issues (especially anti-LGBT), financial and sex scandals, hypocrisy, lack of financial transparency, distrust of religious institutions and leaders, and hostile attitude to young people as major reasons.
The evangelical polling firm Barba found that the three words that non-Evangelicals associated with Evangelicalism were “anti-LGBT”, “Hypocritical” and “Judgemental”. These were also the top three words that younger Evangelicals associated with their own religion.
I would hardly be surprised if former Catholics and young practicing Catholics would use those same words to describe Catholicism.
There have been a lot of studies on the generation gap in various religions, with the main findings being that younger members are far more likely to trust science, and far more likely to support liberal social values, especially LGBT-rights. The generation gap is smaller with regard to abortion.
Unfortunately, these are not the reasons that many leaders of religious groups want to hear.