Kristen Bell is not a “draw” for viewers - She is popular for brief runs on television or streaming, and she seems to be appealing in television commercials, but she’s not an in-demand actor. Hard to make a show popular when your star has limited audience appeal.
The secondary cast were mostly unknowns - When the most well-known member of your supporting cast portrayed a secondary character on the comedy series Just Shoot Me years earlier, things are unlikely to be promising.
Bell was too old for the role - She looked like the mid-20s actor that she was and many audience members never really “believed” her character because of it.
It took time to “get into” - If you have never seen the series, it’s difficult to explain it. You really need to invest time and effort into it and many people simply decided after one or two viewings, not to do so any longer.
The UPN/CW “swap” - The series started on UPN which subsequently went under and reformed as The CW. Many shows were “lost” during that changeover and although Veronica Mars managed to make the transition, it low ratings continued until it was cancelled.


To add insult to injury (https://www.quora.com/Why-wasn-t-Veronica-Mars-TV-show-more-popular/answer/Jon-Mixon-1), the project was given a feature film airing seven years after its cancellation. Unfortunately many of the reasons that the series itself wasn’t successful followed it over to the film and it was financial bomb despite only costing roughly $6 million USD to make. The narrative is essentially dead now as Bell is now 40 and has “aged out” of the role.