Because only Zero, KnK, DEEN Fate, DEEN UBW and Tsukihime existed (yes, I know le ebin "no Tsukihime anime" meme. Let me make a point without referencing it) then. So Prisma was like the second good Nasuverse TV show. There were less people into the manga and nasuverse stuff in general, so it was easier to impress the public. It also helps that Hiro has improved far more than SL have, so SL art is seen as lacking now when compared to the manga (which wasn't the case for the stuff covered in earlier seasons when Hiro's art was rougher).
Now, the manga has a better art style. People know they can expect better from Fate (or have been spoiled) and are into the manga.
So, opinions change. I still think people are too rough on SL, even if they deserve criticism.