Quicken is not an interesting product for people with millions of dollars to manage. They will largely use professional money managers. Quicken is not an interesting product for people with very limited savings and investments, particularly if the investments are largely concentrated in 401K accounts. The natural market for Quicken was individuals and families with significant financial complexity but not wealth.
Over the past fifteen years that market went away. The saving grace for niche Mac vendors is that, insofar as some remnant of that market still lives, it's now largely using Apple products.
In the end, I think the collapse of the American middle class killed Quicken.