Sheikh Muhammad Al-'ameen correctly points out
وليس ضياع أقوال صاحب مذهب، دليلاً على ضعفه مذهبه. لا. وإنما دليل على عدم تبني السلطان لمذهبه. فالثوري وابن المبارك أفقه من أبي حنيفة، وقد ضاع فقههما. وهذا الليث أفقه من مالك (بشهادة اثنين من كبار تلامذة مالك)، وقد ضاع فقهه. وما كان الأوزاعي دون أبي يوسف، ومع ذلك فقد ضاع فقه الأول وبقي فقه الثاني لتبني السلطان له
The disappearance of the sayings of the head of a Madhab is not evidence that that Madhab was weak. Rather, what it is evidence for is that the Sultaan of the time did not accept the Madhab [and use it for legislative purposes].
Al-Thawri and Ibn Mubaarak were both more learned in Fiqh than Abu Hanifah but their Fiqh has been lost.
Al-Layth was more learned in Fiqh than Maalik [some of the biggest of his students bear witness to this]. But his Fiqh was lost.
And Al-'awzaii was not less than Abu Yusuf but despite that the Fiqh of the former was lost while that of the later remained becasue the Sultaan accepted it.