The 250k is messaging to minimize moral hazard. You want banks and depositors invested in minimizing their own risk.
I'd guess hesitancy to immediately declare full guarantee of funds is also due to these concerns (although there may be other reasons as well). The sweet spot here maximizes the appearance of consequences while minimizing actual fallout.
The consequences are the reduction of billions in shareholder value, and we shouldn't be dumb about doing anything that reduces confidence in US banks. SVB shareholders will be down from like 60 bln mkt cap during the pandemic to a good fat 0.
I'd guess hesitancy to immediately declare full guarantee of funds is also due to these concerns (although there may be other reasons as well). The sweet spot here maximizes the appearance of consequences while minimizing actual fallout.