I think there are a ton of reasons that often boil down to tank failures, time, and money.
Tank failures are often either due to lack of research in the beginning, or misinformation while researching. Misinformation is a big one. For every "fact" you read about keeping aquariums, there is an equal opposing "fact". ____ fish is super peaceful and would never harm another creature | ____ fish killed every fish in my tank overnight. ____ substrate is the only substrate that grows plants well | ____ substrate sucks and will break down into mud in 6 months. ____ lighting is the only way to go. | ____ lighting is an overpriced piece of junk. Sound familiar? I see it every day in the forums and facebook groups. It often gets very ugly too - the amount of hostility and tantrums I see in facebook groups alone could put people off to the hobby.
The second one is time. People can't always predict the amount of maintenance a tank will take, and how much time they'll have in the future. I would've never thought this a few years ago when I started my tank (low stress job and no kids). Now, with a busy job and a 2 year old, there are definitely nights where I just don't feel like doing tank maintenance. I still do it because I have a successful tank that is great to watch, but if I couldn't get plants to grow and was having fish issues, it would be another story.
Last is money. This hobby can be expensive. People often try to go cheap and cut corners, only to end up with a failing tank. As a newbie, it's hard to know what you can go cheap on, and what you can't. For example, when I was starting out, I went with pool filter sand and root tabs. Everyone swore that it didn't matter at all. I tried and tried to get a plant to carpet, but nothing would spread. I finally got frustrated and went dirted (still cheap, but I had believed people who said it was a waste of time). Everything else the same, and I had a full carpet of HC in 2 months. Same thing with co2. I spent more money on alternatives to a pressurized system than it would've cost to just get one to begin with. I never had much luck with any of them either.