Same reason the football has not used a microchip in the football for yardage. They want to hang onto tradition, but have to negotiate every single change with the Owners, Players, and Ref Unions. With all the technology we have, by simply placing a chip in a ball and using high speed cameras, they could eliminate all mistakes on First Downs, Touchdowns, Out of Bounds, Field Goals, and Safeties.
Tennis has immediate replay of every ball and where it hits on the court. They know with extreme accuracy whether it was on the line, inbounds, or out of bounds. Baseball could do the same for pitching, foul balls, and safe calls, by placing a sensor in the balls and each base. It wouldn’t eliminate the need for refs and umpires, but would make their jobs easier. I am all for tradition, but I would much rather just have the right call. Instant Replay is great, but leaving it with the possibility of a “Stands” call or incorrect judgement, is unacceptable when the technology already exists. If the pylons can have cameras in them, they can easily place a sensor in them with a chip in the ball.