I don't know if there was a specific reason. Lancer was my absolute favourite pony as a kid and I specifically had my mum write to Hasbro to ask them if the three Tales boys existed as figurines but we never got a response. For years, I hoped I'd find him at a car boot sale - I'd read on internet pages that he and the other boys didn't exist but refused to believe it. Many tears were shed over the lack of those boys when I was a little girl!
The whole Tales thing was rather strange really. Hasbro had obviously decided to end the pony line, MLP production for the states had already been cancelled before the show even aired. So why the heck did they spend all that money making the TV series in the first place? That show had so much potential for further seasons and accompanying toys (I'd have loved to see a Schoolbus or Ice Cream Shop playset, for instance, let alone all the additional characters in the show!) The only specific Tales toys they made were the seven main female characters and those were never available in the US and Canada. Eventually the MLP comic here in the UK went over to using Tales characters, but they were all wrongly coloured as if even the illustrators were not given pictures of the characters to work from!
Hasbro missed a great opportunity in my opinion.
It's like they'd just decided "it's time to throw the MLP line away and focus on Nintendo and video gaming!" and were determined to prove MLP was a flop. Of course, I realise that's not the case, but it certainly looked that way! >.<