Captain Marvel has a history of either selling poorly or having a case of writers not knowing what the fuck to do. I'll give you a run down.
- Mar-Vell: Put on hiatus due to low sales, brought back and then cancelled for good again due to low sales. Eventually after a few appearances in other titles died of cancer in Marvels first Graphic Novel.
- Monica: Introduced as Vells replacement in her own one shot and then made a member of the Avengers. Fell into obscurity and gave the title to Legacy, Vell's son. She changed her name to Photon. Now currently goes by Spectrum.
- Genis-Vell: The genetically engineered son of Mar-Vell, Genis started out as Legacy, his series was quickly cancelled. Years later after Avengers Forever and a few cameos later it was relaunched by Peter David... twice. Cancelled again due to low sales. Genis would go on to appear in Thunderbolts as Photon, again leaving Monica to change her name.
- Phyla-Vell: Alternate reality reboot version of her brother Genis, Phyla co-starred in PAD's second Genis run. Eventually she'd return in the Post-Annihilation cosmic books, undergo a few name changes and then die.
- Khn'nr: A Skrull who was brainwashed into thinking he was the true Mar-Vell lost in time. He eventually came to terms with who he was but refused to be a puppet of the Skrulls, choosing the life of a hero instead. His mini series sold poorly and he ended up dying passing on the mantle to Noh-Varr.
- Noh-Varr: A kree bad boy who was known basically for being a bit of an asshole in his Mini-Series Marvel Boy eventually took on the title of Captain Marvel during the Dark Reign. He quickly tossed it away when he became the hero known as Protector. Last seen he was an idiot himbo who likes 60's music.
So there you go. Low sales and bad ideas.