Well, my best bet would be that the producers of the Pokémon Anime wanted to prolong the series. Plain and simple.
As you know, Jessie provides a soothing musical number with her Pokemon doing a little dance. Which is nice enough as it is.
However, part of it might be due to something that happened in an earlier episode.
Way back in the beginning of the Diamond and Pearl series, Dawn and her Ambipom did a performance where Ambipom was completely enveloped by the stars from it's Swift attack. Piplup and Buneary have a lot of bubbles floating around that may partially block Buneary and especially Piplup, seeing as they share a color pallet. Even though it ends in a classical shower of sparks, it may have been difficult to see, and maybe difficult to judge.
However, this is only a theory of mine.
Also, though I've forgotten exactly how many badges Ash had, it may have been so that they can continue to have contests while Ash travels. In addition to that, Ursula won't have had that ribbon, possibly putting her out of the Grand Festival. Dawn winning the next one guarantees her a spot in the Festival, and then the Princess (I forgot her name) with her Togekiss wouldn't have won the Contest and given Jessie her last ribbon, getting her a spot in the Festival as well.
It's a puzzle that fits everyone in the plot nicely.
Anyways, hope that answers your question.