Because of cap space and the money Tampa has tied up at receiver. Godwin, according to PFF, is the top WR on the market. That's over Golladay, Robinson, Corey Davis, you name it. He's going to command serious dollars somewhere and Tampa has as a ton of money already invested in Evans. They might franchise Godwin, but they'd be lacking for resources then at other positions, and both Scotty Miller and Tyler Johnson are up and coming receivers that might be ready to step in in his absence. Capella, TB expert on the board, predicts they'll indeed franchise him, but I haven't seen that in any of the publications. Barnwell had him going to New England, which got me worried. He'd draw double coverage all day and I'm not sure they have a QB that can get him the ball.
Anyway, I'd keep my eyes peeled for a move for mega money.