One reason someone could argue against allowing renunciation of citizenship is to prevent citizens being coerced into giving up their citizenship. That is, as an explicit counter measure to make requirements to give up citizenship as a precondition for taking on a new one impossible.
To give an example, person X might be of single nationality B, but born in country A, to a family which cultivates its heritage from country B. There are strong incentives to take on A's citizenship (democracy/participation, protection from deportation, etc.), but A doesn't allow multiple nationalities. Now if B allows renunciation of citizenship, external factors practically force X to renunciate B's citizenship. But if B blanket disallows renunciation of citizenship, free and democratic state A can't legally hold that against X, and will allow them to take on A's citizenship anyway, maybe dependent on some conditions (like never using their second citizenship ever again) that will be impossible to enforce in most cases.
In general, a country which wants to give its citizens maximum freedom can disallow renunciating citizenship and support its citizens in ignoring other country's single citizenship laws to enable their citizens to live in other countries for extended periods of time without having to lose part of their identity or practical advantages granted to those of the original citizenship.
A second related motivation such a policy could have is to encourage multi-citizenships with the goal of eroding national borders in general, promoting cross country freedom of movement and over multiple generations fostering more and more of an international mindset like can already be found in some EU 'citizens' today to make sure two countries having exchanged enough citizens could never go to war with each other ever again.