Why did Garry Kasparov retire in chess

Why?

Because he started losing more to the guys at the top.

He went into politics. Probably more meaningful anyways.

By the time of his retirement, Kasparov had witnessed a generational shift in chess, with young and upcoming talents like Magnus Carlsen and others starting to make their mark. The rise of computer chess and the influence of engines like Deep Blue (which he famously lost to in 1997) also signaled a new era in the chess world. Kasparov may have felt that it was the right time to exit the competitive scene gracefully.

I think he played for 5 or 6 years after the match loss to Kramnik.

Same reason most people retire: age and diminishing capacity/energy.