I think you missed the point. You don't choose the Church for priest being rich or poor but for what is offering you.
Orthodox Church is offering you right belief , protestant Churches is offering you right and wrong beliefs. Should you go from right belief into wrong belief because priests are poor or rich?
Orthodox Church is offering to you the sacrament of confession that erases sins being like a new baptism in this aspects, Protestant Churches may not give you this. Are non ordained pastors allowed to perform the sacrament of confession? I don't know. How do you clean your soul without confession. Is it better to remain orthodox and spoil the soul then clean it then spoil it again then clean it again or go to protestantism because priests are rich and continue spoiling the soul at a lower speed without even cleaning it again? How is a shirt spoiled and then cleaned and the spoiled and cleaned compared with a shirt never cleaned even if you show care toward it? In my understanding in all Churches before 1600 only ordained people were allow to perform confession, non ordained pastors not being of these.
Orthodox Church is offering you Holy Eucharist or food for eternal life after teachings of Apostles see Holy Liturgy of Apostles James, Thomas, Peter , Mark....
The Orthodox Church is offering to you prayers even after your death for FREE, Protestant Churches leave you alone after death. Do you want to be left alone because some priests are rich? Yes even Jesus prayed for dead Lazarus and had effect. More than that Holy Liturgies of Apostles had prayers for departed so wrong belief is the cause for missing prayers for departed.
Orthodox Church is offering to you ordained priests allowed to perform Holy Liturgies,confessions according with Church teachings, Protestant Church may offer you non ordained laical men named pastors.
Orthodox Church followed teachings of the Apostles, Protestant Churches protested and reformed the teachings of APostles, Apostolic tradition, would you want to be part of such protest and reformation because some priest were rich? Fort example they replaced Holy Liturgies on Sunday designated after teachings of the Apostles with a sermon. If the changes were limited only to not put clergy to inthronate a king I won;'t say anything anyhow the reform cut deeply and replaced the teachings of the Apostles from which Bible is only a part. Do you wnt to reform teachings of Apostles like Bible because priests are rich? Who gave the authority to reform leaders to reform and protest the teachings of Apostles? NOBODY. And they error out in many aspects and they introduced wrong teachings. If they retruned to PAtristic teachings where are Holy Liturgies and ordained priests in Protestant Churches?
And I can continue. Church is not only about not drinking and being nice, is about more than that.
I think the root cause for people remaining or leaving the Church is God's will. Visible causes for leaving may be sin and religious illiteracy.
Probably the main cause for orthodox countries remaining orthodox was these countries being busy with wars for existence, they did not have time to think much about religion and start changing based on their understanding and to start to protest and reform so they remained with Apostolic teachings and with mother Church the Eastern Orthodox Church.
has there been any studies done as to get to the root cause why these people leave the church? especially leaving for something like Evangelicalism or Pentecostalism yikes
There are tons of good reasons for anyone familiar with the way Orthodoxy works in some parts of the world. Overweight rich clergy riding around in their Mercedes with $100,000 watches while their flocks scrape together a living and still donate to build beautiful churches. Not understanding anything being prayed or read in the churches because of the use of archaic languages that are nearly if not totally dead. Almost nonexistent catechesis and rampant superstition. The charitable outreach of Protestant missionaries and the hope of a better life that can bring with it (education, food, prayer support without financial motivation required [i.e. no simony for sacraments]), etc. Church administration being controlled by the state and then being used to control the people. There are lots of good reasons to leave.
Try talking to a destitute peasant about church fathers and church history and such versus a better life and see how many care. I suspect not many. It's mainly fidelity to the traditions of their ancestors that keep most in the church. And that's not a bad thing.