[joezydeco]

It's a tricky thing. From my experience, the capacitive sensing chip vendors (e.g. Broadcom, Cypress, Atmel) want to move units but don't want to spend the time working on the necessary physical integration to make the sensing pad work correctly. A trackpad is really nothing more than a phone-class touchscreen without an LCD behind it. There's a lot of sensitivity tuning and noise rejection that needs to be performed to refine the performance.

It's not their field of specialization, and they don't want to manufacture the finished components themselves. Apple obviously has taken the time and care to get it right. A lot of others...it seems like they went with the reference design as being good enough.