Neoprene rubber gives a good balance between having both oil & outdoor resistance. If those cracked washers were maybe EPDM rubber, potentially the best outdoor rubber material, but it gets swelled & split by petroleum based oils & greases. Any grease or oil used on the spindles could adversely affect EPDM if thats what rubber type those were by any chance.
All modern rubbers lack lots of the "good stuff" that made them work as intended years ago. Its mostly down to the ongoing banning of various harmful chemicals. Then theres also potential to make a cheaper rubber compound thats highly filled with poor ingredients, i.e. less actual rubber polymer. Easier for the factory to mould but poorer end use. The best rubbers for end use are always harder to work with.
Another factor is incomplete curing, if processed for too short a time in order to get production targets out.