Because it's old (over 10 years old), works well, battle tested and practical.
Trello allows for great collaboration and informal planning, even for daily life and for a single person. It allows capturing great deal of information under a card and showing a great deal of information without overloading your senses.
Actually I have an integrated system which enables me to collaborate with many people on Trello, and my knowledge base is on Evernote for a very long time now.
I consider moving from working systems to other stuff just because it’s new and hip harmful.
Trello allows for great collaboration and informal planning, even for daily life and for a single person. It allows capturing great deal of information under a card and showing a great deal of information without overloading your senses.
So, in short, "It works (TM)", and works well.