I can only answer your question from personal experience and not theoretically.
My 2 brothers and their wives left The Church in the late 90s and joined one of those mega-churches founded by 2 guys (non-practicing) from Harvard Business School who decided to create churches based on what people wanted. For instance, they have a Starbucks style coffee bar when you come in and you can take your mocha frappacinos and carmel lattes into the service. They kick back and get entertained for a couple hours in a big amphitheater, where Christian pop bands perform and a lively, likable, good-looking preacher challenges them with easy to follow/how-to instructions for applying biblical messages to middle-class daily problems.
The congregation is broken up into small groups known as “smile groups” (don’t ask me) where they meet in each other’s homes several times a month for potluck, fellowship, advice and support. They have small groups for women, men, those overcoming addiction, they allow divorce and have groups for divorced people to help them find a “good Christian guy or gal,” they employ Christian auto-mechanics who will fix your car and give you an honest estimate, if you’re out of work, they offer job placement services.
I asked them why they left Holy Mother Church and they said that, for them, Sunday Mass felt like nothing more than punching a time card, click…I went, and that was that. At their new church they have instant community of like minded people who want to “truly live the Gospel” and support each other. I guess it never dawned on them that they could have found all of the above, in different ways, in The Church, but that would take work, the hard work of study, prayer and sacrifice. No, this is the easy American Way. Though, I will say, pastors don’t often encourage or challenge their parishioners to learn during the week on their own or grow in their prayer lives.
Other people I have met who choose Protestant churches over Catholic are those who take issue with the moral teaching of The Church. This type quite frequently ends up in the Episcopal/Anglican church where they can get Catholic-style worship, without the dogma and moral teaching.
One silly woman I met told me she would never join the Catholic Church because “the priests drink from gold goblets.” When I explained that we use precious metals because we believe communion to be the actual body and blood of Our Lord, she shrugged, nothing would change her mind.
Those are just a few examples I have encountered over the years.