I heard an interesting take on this at all things a music festival that my piano playing son attended a few weeks ago. This is not my area of expertise, and I wish I had better notes, as the faculty who spoke were experts and I wish I could summon them into this post.

This is a summary from the Baroque Festival that I attended with him: In the early Baroque period there were many of the "classical" instruments and notations of music coming about. The harpsichord and organ dominated the "keyed" instruments. The music of the time before was dominated by "high" music (church funded, religious minded) and "low" music (tavern/minstrel/common people). The Baroque saw an explosion in the patronage of the middle and upper classes investment in music. Harpsichords became fashionable in French and Italian manors. Standard music notation helped spread music and musical ideas to a much wider audience. Then the invention/innovation of the piano became a trans-formative instrument that went from an oddity to a necessity during the era.

Moving to 19th, 20th, 21st centuries, pianos spread all over the globe. While most of the instruments still survive and have a following, you would be hard pressed to find an organ outside of a church and how often have you seen a harpsichord outside of a museum? Part of the reason for the German composers having some staying power is that they were some of the early adopters of the piano and wrote many famous pieces for the piano. France and Italy in particular held on to the harpsichord pieces more than their German counterparts. A large portion of the Baroque era piano pieces are/were written by the authors you mentioned. While that is not all that they composed, it does account a bit for their fame and staying power when you have generations of piano students learning to play their pieces.