Shortest plumbing and tradition.
Being seen by the pilot is not relevant - consider low wing aircraft, unseen pitot either side.
Being seen by the Pilot is very relevant. Would you want to know if you've left the cover on? Or you have hit a bit and it's hanging off the wing?
Traditionally aeroplanes were high wing with a good view of it. Prior to the pitot tube they had venturi's on the left side of the fuselage.
On a low wing aeroplane such as a Bonanza etc I never had a problem spotting the Pitot from the hot seat.