Because they have more valence electrons to play with. Earlier elements have too few outside the core, and later ones tend to have their $d$ electrons sinking into said core. I will add that gold is known up to +5.
– Oscar LanziOct 14 '22 at 16:45
1
You say they show more number of oxidation states, and I say you got lame data. That's only most common ox. states.
– MithoronOct 14 '22 at 16:53
related https://chemistry.stackexchange.com/questions/67510/why-do-heavier-transition-metals-show-higher-oxidation-states
– MithoronOct 14 '22 at 18:54
https://chemistry.stackexchange.com/questions/61353/why-do-osmium-and-iridium-have-the-most-oxidation-states-of-all-the-elements
– Nilay GhoshOct 15 '22 at 02:31