The economic development forecasts used to get approval to build public facilities like stadiums, arenas, convention centers, etc. are almost always exceedingly optimistic. That's not to say they are always bad deals for cities, but more of them are bad deals than good.
This plan had significant private investment pledged ($1b from the Royals and $300m from the Chiefs), but $2b for a downtown ballpark and $800m for renovations to Arrowhead? That probably seems a little steep to the average Kansas City resident, especially considering that tickets to the new stadiums are almost certainly going to be more expensive than the current tickets. I'd have a hard time voting to subsidize stadium construction for a team I felt I couldn't afford to see in person.