A plausible reason would be a Near Death Experience, for the country itself, such that secret negotiations and the corruption that naturally goes with it destroyed them economically and caused a major coup attempt that nearly turned them into a dictatorship. In the shooting war that followed to prevent this political disaster on top of an already devastating financial disaster that left them in a great depression, the government was reformed, as for the people.
Part of that reformation was absolute public meetings, there are laws in place so if any politician meets with any other in private, both are guilty of a serious crime (mandatory sentencing of 3 years in prison, forfeiture of 100% of assets, and barred from politics for life). The people created a policing organization, the Congressional Collusion Investigation and Prosecuting Force (CCIPF), and they have draconian power to enforce it:
Politicians cannot be friends with each other and make deals on the golf course, they will go to jail. The standard of proof is simply "preponderance of evidence"; the lowest bar (in the USA) for a conviction.
Why would anybody become a politician? It pays very well, as much as, say, a surgeon. Plus half that amount for life after holding office; because they will also be prohibited for life from taking any other paid job or form of income, once elected. While holding office they get CCIPF bodyguards, they travel, they decide matters of national importance. They can have family and friends, they just have to be careful. But one of their official roles is campaigning, arguing for or against laws in the media, etc. Their CCIPF bodyguards are assigned and cycled at random; and also carry 24/7 recording equipment, the recordings are open to the public, and can be used in either prosecution or defense of any charges of collusion (e.g. accidentally shaking hands with another politician at a wedding is not collusion.) And CCIPF officers can wear GPS trackers when on the job, so they can all have electronic proximity alerts if any other CCIPF officers, assigned to any other politician, are anywhere near their charge. Communications could let them negotiate sufficient buffer space (or alertness) to keep them separated.
The CCIPF will give them fair warning if anybody new in their life has political ties, or anybody already in their life develops new political ties. An honorable politician can make an upper class living for life, the job is relatively easy, and there will be no shortage of candidates.
The CCIPF is similarly constrained: Investigations of politicians are publicly assigned to the CCIPF officers by lottery; and their own lives must be an open book, too. They can never, ever be politicians or involved in politics themselves; joining the CCIPF, even for six months, bars them for life from holding any elected or governmental office or even participating in a campaign or advocating for a particular politician, bill, or scheme. They cannot express an opinion between pro-choice or pro-life; they have no political free speech. They are allowed to vote, in secret, but revealing their vote is a crime. Every dime they earn or spend is recorded and public knowledge.
Further, the CCIPF is funded independently by their own tax which politicians cannot touch, short of a complete rewrite of their Constitution.
Further, the CCIPF itself is incapable of making any law or rule for the citizenry. It has its own court system with CCIPF judges, also assigned by lottery; and trials and evidence for collusion between politicians are public and the jury deliberations are open and recorded.
Who enforces the laws against the CCIPF? The CCIPF; they are organized into a dozen independent bases in the country (like military bases). New recruits are assigned at random to one of these. Charges against CCIPF members are given a trial by CCIPF judges chosen at random from another base with a jury of their peers; one from each base. Other rules like that; like the USA military, the CCIPF is its own sub-culture, a semi-democracy with its own courts and rules about how to conduct itself.
Make the country's wounds fresh, this all happened, say, 150 years ago, and people still honor the names of their relatives that fought and died to create the CCIPF. There are multiple national holidays celebrating various milestones in that "War On Corruption", statues to the heroes and first generals of the CCIPF, the first fallen in the name of Honest Government, and so on.
That is the "social element", the CCIPF is seen like soldiers: Heroic, self-sacrificing, and utterly honorable; they are trusted because the structure leaves little reason to doubt them. Our soldiers protect us from foreign threats, the CCIPF protects us from being taken over by our own mentally ill citizens that have no conscience and seek only self-enrichment and megalomaniacal power, fame, and being feared.
Even if politicians truly believe the open negotiation law is stupid and crippling, they do not violate it, they learn to work within these constraints because they have a good life if they stay within them, and there is too much to lose and not enough to gain by violating them. The uncontrollably greedy or megalomaniacs best not apply, they can exercise those traits in the corporate world. Politicians will be people happy with the generous salary, retirement, and other perks of being in the top management tier of the country.