Because St Pete's leadership (kudos to the new mayor being less of an arsehole than the last guy, but ultimately the council is the judge in this decision) are holding on to the fact they think they'll be able to get some magic bank vault full of coins to Scrooge McDuck backstroke through to allow the rays to LOOK to leave. Yes, the Rays and MLB and sports owners in general are good at pulling the wool over the eyes of municipalities in order to get Joe Schmoe to pay for his grand stadium build, the St Pete leadership seems at times like they'd rather the Rays move out of the area than concede the fact that moving to Tampa would likely save the franchise from bolting away to [insert popular destination in need of MLB here].
Supposedly last time, the Rays were not offering enough in incentives if they were to find a suitable Tampa location, specifically in helping to cover the costs of demolition to the Trop...I can see where St. Pete would like the Rays to pay a few million dollars to turn the Trop into a nice piece of property to entice future property owners to buy...the Trop is built fairly low around that creek, and just the design has a lot of multi-level going on, so I can see where filling the sloping parking lot and stadium site with a LOT of fill would be expensive. Expanding downtown/Central Avenue area would be good for St. Pete, as they seem to be going for a hip, cultural, artsy type of downtown. I have heard Jabil, a big sponsor of the Rays, also would love the Trop site.
The Rays aren't staying past 2027 regardless, so St. Pete really isn't going to get very far demanding a bunch of money if they move "as if they never did"...that's on St. Pete to deal with, and it's not like millions of people going to Rays games are staying in Pinellas hotels, as it's a fairly local fan base.
That's my take. The population density in many of the Tampa areas should more than keep the current attendance figures and might help boost them during the weekdays, when they really do struggle with people trying to get out of work and manage some traffic to get to the Trop. Traffic is inevitable in a region that isn't very effective at public transportation.