According to viral threads, the straw cracks in multiple places along the sides almost as soon as it's popped into a drink, making it impossible to finish a drink.
The original straw was white with the iconic yellow and red stripes and was noticeably thicker and wider than most standard straws. The new straws are thinner and clear.
Sustainability may be one possible reason for the switch — but that doesn't seem likely, as the new straws are also made from plastic. It may also be to save the chain money.