They were part of the Senate culture and didn't really understand the Executive Branch.
I've been a member of local legislative bodies. You are not really administrating; you making collective decisions. Even in local governments, where you can work with local administration, there is a difference.
Recently, most presidents were governors, who administrate thing. The exception was GHW Bush, who ran a large government agency (and a business).
It's not only the lack of understanding of the executive branch, but also how one portrays issues. A legislator generally looks to their base in definining and explaining issues. Moving to the executive branch requires a more inclusive perspective. In speeches one will often note a level of specifics and details from legislators that are not present in executive presentations.