Seems like it was PBS' decision to end it, which is fukked up if you ask me. It was one of the longest-running cartoons of all-time, in the same company as The Simpsons and South Park. You don't treat one of your most well-known shows like that.
With that being said, this isn't a huge surprise. The show had been slowing down in terms of production for years. I remember when I was younger, they did ten episodes every year in April and October. Then they started making three episodes a year. I also think moving to flash animation was the beginning of the end. The show had a distinctive, soulful look to it and flash animation took away a lot of that soul. I know it was to cut costs, but it made the newer episodes really hard to watch.
I saw that tweet from Binky's voice actor a while back and I thought it was weird at the time, but looking back, it makes sense. The least PBS could have done was give them a series finale, but at the end of the day, you can't erase 25 years of hard work. Rest in peace to one of the GOAT animated shows.
I notice that they don't just come out and say shows are over. They just quietly stop ordering new episodes and expect kids to figure it on their own. Nickelodeon pulled the same shyt back in the day. Animated shows weren't allowed to have series finales. They would just end and reruns would be shown constantly. They used to do the same thing with live-action shows, but then they stopped.
Going back to PBS, I recognize the pattern. Martha Speaks was just cancelled with no explanation. I remember looking it up and finding out on Wikipedia. There was also a three-year hiatus with Cyberchase. I'm guessing PBS was done with the show, but then changed their mind and revived it.