In my view it was political cowardice.
Obama was determined to make a deal with Iran and republicans were afraid to get in a fight him to assert their constitutional authority to ratify (or not ratify) treaties.
So, in an effort to disguise their surrender and make themselves seem relevant they passed the Corker-Cardin bill that basically confirmed their agreement with Obama that the Iran treaty was not a treaty.
It allowed Obama free rein to negotiate a treaty and do an end run arounfd the US Constitution by calling it an agreement. All with the blessing of the republicans in the senate and congress.
In practical terms, the bill guaranteed Obama a victory but gave the senate and congress a way to pose and posture and make gruff statements while actually doing nothing at all to effectively oppose another Obama usurpation of their constitutional perogatives and authority.
AKA it acted as a pressure relief valve to assist the GOP in keeping the illusion of two parties alive.
Folks might want to ask why there is so much buddy buddy off the field. Barry and Bhoner golfing...Bush Clinton ‘brotherhood. Bush giving Clinton medals...
Is that how to polar opposite party members act? Or is that how people with a shared vision act? Anyone ever see Bloods and crips together without gunfire? Nope. So why would these two criminal gangs be any different? Answer? There is no spoon.