Why was he not an attourney?

His father was- and unusually ,IMHO- Mr Gardiner did not become a member of the same profession as his father,but became a tradesman( never specified what exactly, but described as a respectable line of trade.)in London.

He obviously had more education than either Mrs Bennet or Mrs Phillips;

."Mr. Gardiner was a sensible, gentlemanlike man, greatly superior to his sister, as well by nature as education.
Chapter 25

But he did not turn to the law.Instead someone unrelated to the family, Mr Phillips, who had been Mr Gardiner seniors clerk,

succeeded him in the business
Chapter 7

Why? Mr Gardiner has obvious commercial business sense,he is sucessful in his trade, so he seems well qualified for the role of an attourney.

Perhaps money was a problem?Mr Gardiner senior might not have been prosperous enough to enable his son to suceed him in the firm, and needed to be "bought out" by his clerk, Mr Phillips?

Perhaps Mr Gardiner aspired to be a barrister like John Kngihtley,but could not afford the rather expensive training?Perhaps he just did not want to be a lawyer( understandable;-)...)but it's always puzzled me that tha family firm did not remain in the family,when there was a sensible,competant and educated son to following in his father's footsteps.