So, consider the situation. They had just launched Thomas in the US, as part of Shining Time Station. There was obviously the issue of no female-identifying engines. Rick Siggelkow saw the Daisy stories and character and instantly said 'no. this will make the problem worse'
He was right -- because no matter how you look at it, the character design, while humourous, is oddly sexist. Overly feminised faces etc. But he came to a compromise with Britt - and they decided to dub the episodes ONLY for video, and release Daisy toys with the STS brand.
Rick was very involved with Series 3 and many of the stories were his idea (see my interview with him, on YT) -- so this is when he decided to address the issue of no female engines that weren't a stereotype -- via Mavis, who was changed from her book appearances. Mavis is great.
So why did Daisy re-appear in Series 4 and 'Thomas Tales'? Because while Rick was involved at this point, he began to have less and less say for various reasons, until he got tired of it and left for the BBC. Simple as that.
In conclusion, I'm not just saying that I agree with Rick because he's my mentor and friend, but he definitely had a point. The character design was sexist, arguably the book illustrations were LESS so! If someone wants to put this on the wiki -- feel free, but cite this tweet.
Also disclaimer; as long as you acknowledge the source material is problematic, liking the episodes isn't particularly a bad thing. It is possible to enjoy something while acknowledging its faults. I am not an advocate for censorship, but I agree with Rick's decision.
OH and Series 3-- that's why Daisy did not appear lol. Since the stories were made to focus on the US and Japanese (who didn't really mind either way lol) audiences.
It’s a one off line lol