Answer:
To ensure that they are impartial from the influence of other branches.
Step-by-step explanation:
The federal judges are appointed by the President (Executive branch) of the government and confirmed by the (Legislative branch) of the government.
If Federal judges are serving limited terms, their position can be influenced by both the legislative branch or the executive branches as a bargaining chip in exchange of a certain ruling. (For example, the legislative branch could force the judges to overlook some violation of laws in their proposal or they will be removed from their position otherwise)
By giving the judges a lifetimes appointment, we will minimize the other branches' influence,. The judge could make a ruling against any of them without feeling threatened that they will lose their position.