Jenna just couldn't be developed into any character archetype that wasn't already used. We already had the jock (Francine) and the quieter kid (Fern) - Jenna usually fell into both of these archetypes, without much else to offer. Her last role (that I've seen anyways) portrays her in a more kindly light (the nuts episode), but it still offers nothing that more established characters couldn't pull off. I do think after 16+ seasons, Arthur could do with a change-up from the old Arthur gang - but that probably won't happen. The core characters are just too well ground into the show to really retire any of them.
I don't think she's a bad character, even if she was brought up by provocation in her two main episodes ("The Good Sport" and "Jenna's Bedtime Blues").
As for the bedwetting habit, that's fairly low-key in the Arthurverse. Buster is now known to suck his thumb, so what has Jenna got to lose?