In addition to the fact that plenty of progressive, accomplished women politicians were more than capable of kicking off a convention that's focused on women's rights, many of us have valid concerns about some of Sanders' stances.
For example, he recently supported an anti-choice candidate running for mayor of Omaha, Nebraska. He also penned an essay in 1972 in which a woman fantasized about being "raped by three men simultaneously." Although this was admittedly decades ago, many women (especially those who have survived sexual violence) struggled with the fact that it was overlooked by the media.
Of course Sanders certainly has a number of policies that benefit women and he's not the enemy, but the fact remains that women are woefully underrepresented in politics and this was an excellent opportunity to give female politicians a platform.
The organizers later apologized for the "hurt and confusion" caused by the initial announcement and stated that Rep. Maxine Waters (CA) is the event's headliner. But many felt the announcement didn't go far enough and that by having Sanders kick off the convention, lesser-known female politicians were being overlooked.
According to Sanders' statement today, he no longer plans to attend the Women's Convention at all: "I want to apologize to the organizers of the Women’s Convention for not being able to attend your conference next Friday in Detroit. Given the emergency situation in Puerto Rico, I will be traveling there to visit with San Juan Mayor Carmen Yulín Cruz and other officials to determine the best way forward to deal with the devastation the island is experiencing," he wrote. "The U.S. Congress cannot turn its back on the millions of people in Puerto Rico who, four weeks after the hurricane, are still without electricity, food and running water...My best wishes for a very successful conference."
As Politico correspondent Edward-Isaac Dovere notes, this seems like a fairly blatant attempt to duck a controversy. After all, the situation in Puerto Rico has been an emergency ever since Hurricane Maria made landfall one month ago.
Nevertheless, Puerto Rico desperately needs the help of as many politicians and civilians as possible. Sanders' presence could certainly be beneficial to the devastated U.S. territory and its residents.
The Women's Convention, on the other hand, will be just fine without him.