I can understand your question. I may not agree with John Piper all the time, but when I would like a 21st century Reformed position on an issue or a Bible verse interpretation, especially when there are multiple viewpoints even within the Reformed camp (as you know, there are variants such as Hyper-Calvinism, Neo-Calvinism, Christian Reconstructionism, etc.), John Piper is usually my initial go-to source to get an even-handed Reformed perspective before going deeper or broader.
His extensive Desiring God Ministries web resources which include Articles, Sermon collection organized by Scripture or topics, Podcasts, In depth exegesis, Books by him and others, etc., prove to be:
- professionally written: using college level English composition principles, respectful of the audience (not patronizing nor talking above them)
- exegetically responsible: don't overstate what the Bible says, using historical-grammatical method preferred by everyday conservative Christians (vs. liberal theologians)
- theologically informed: clearly separating the Bible interpretation step and the theologizing step, leveraging his doctorate level theological education in University of Munich
- historically informed: bringing in church history and historical theology when relevant, by citing what theologians in the past (Reformed or not) said about an issue, or by contrasting the Reformed view with other denominations (past and present)
- pastorally sensitive: bringing in his decades of being a senior pastor who is aware of how people really apply Bible verses to life
- scholarly oriented: by providing references to peer reviewed books when necessary
- a known commodity: which naturally comes with fame, as more theologians interact and write about him, earning him a well-referenced Wikipedia article section on his theological views
- consistent viewpoint across the board: most of his articles reflect uniformly his well-considered theological stance, which he calls Christian Hedonism which a lot of Christians nowadays can relate, since everyone wants joy and happiness but in a Godly way, not in a materialistic or a prosperity gospel kind a way. Some claim that prosperity gospel is among the top issues that John Piper has a visceral reaction against.
- aligned with C.S. Lewis's view: making Christians who are already persuaded by C.S. Lewis's arguments more comfortable with him (see his article Was C.S. Lewis a Christian Hedonist?)
- informative to common questions: shown by his respectful, sensitive, and Biblically responsible answers to common questions (see his website section Ask Pastor John), not hiding his own opinion but clearly separating it from the Bible and the Reformed tradition. Therefore you can trust that he is not pulling the wool over us.
(Note: I'm only endorsing the web resources authored personally by John Piper, as some articles / sermons in that website don't rise to the same level of quality)
Other reasons for his popularity (from @curiousdannii):
- Piper is very cross-cultural mission oriented.
- His multiple decade commitment to the charismatic Passion Conferences where many attendees are 18-25 is notable, as it is a place where Reformed teachers would not get as much of a hearing.
- Contributing to his appeal is also simply his longevity in ministry. Desiring God (the book) was first published 35 years ago. By contrast, Tim Keller's first well known book, The Reason for God, was only published in 2008.