Hi there! This is an unexpectedly relevant question for this fine, fine 75th Liberation Day!

There are several stacked reasons which kept the 'Breda Four' or De Vier van Breda in prison for such a long time after the Second World War. For each individual there are important reasons, which I will not go into all that much (except for Kotalla's case), but collectively there are more definitive factors that extended their imprisonment.

For starters, The Four of Breda only acquired their moniker about a decade after the war. During late April and early May of 1945 all of them had been arrested or captured on Dutch soil, and between 1945 and 1952 they had been incarcerated in the Norgerhaven Prision in the town of Veenhuizen. At first they awaited their trial there, thereafter their sentence, and finally the execution of their sentence. Each individually they had been proven war criminals, accountable for the deaths of Dutch citizens of all kinds; resistance fighters, Dutch Jews, bystanders, collateral damage etc.. They were each individually notorious for decisions they made or actions they undertook. In finality, even after trial by an independent judge in the overall shaky court of law immediately after the war, they were all unanimously sentenced to death for their crimes. As early as December 1949 these sentences were passed, but not executed.

The reason for the staying of their sentences is somewhat complicated. In 1948, wartime Queen Wilhelmina, had abdcated, and passed on the throne to her daughter, Princess Juliana. Juliana was not cut from the same cloth as her mother. Where Wilhelmina gained a reputation of "being the only man in the Dutch government-in-exile", Juliana was much 'softer' and intellectually engaged with the way she wanted to shape her own rule. Wilhelmina's bullishness suited the volatile political atmosphere of the war, and gained her massive popularity in the postwar Netherlands. Juliana had from the start of her reign vowed to push back against that, and with humanity and humility help shape a newer, brighter future, devoid of old enmity. Her personal convictions -- which were in essence almost Christian-socialist -- played a major role in many of her less popular decisions. One of the first unpopular decisions she made upon ascending the throne, was the commuting of death sentences of many (war) criminals, whose trials finished just as the 1940s drew to a close.

The Breda Four were among those war criminals whose sentences she commuted from death- to life sentences. Although the law, the courts, and public opinion were clearly geared against this, Juliana's appeals eventually won out. Although no court of law would actively ignore the Queen's clemency, judges and legal scholars did very actively work to interpret these staying of executions creatively, resulting in harsher interpretations of 'life in prison' than we are used to seeing nowadays (where one is eligible for parole after sitting out 2/3rds of a sentence etc.).

You should imagine that public outrage over this fact was rather double-sided. On the one hand, the Dutch in the early postwar really rallied around the House of Orange in their newfound freedom. On the other hand they were out for the blood of the Breda Four, and especially Kotalla's, as he had built up an infamous reputation as the Ortskommandant of Kamp Amersfoort, one of several concentration camps in the Netherlands. Their transfer to Breda in 1952 produced their nickname/moniker, and throughout the 1950s and 1960s, several cabinets and legal scholars would very actively challenge their sentence, the particulars of their inprisonment, and the greater role of their case in Dutch-German relations in a Cold War that was rapidly heating up.

Now, I could go into the personal cases of each of these Breda Four, to explain their significance in the Dutch context, but as said, that might be beyond the scope of your question, as well as beyond the scope of the 10k that I have to explain all of it..

The main sources I used for this is as follows;

Hinke Piersma: De drie van Breda. Duitse oorlogsmisdadigers in Nederlandse gevangenschap, 1945-1989. Amsterdam, Uitgeverij Balans, 2005.

Henk te Velde & Donald Haks: Oranje Onder - Populair Orangisme van Willem van Oranje tot NU. Amsterdam, Prometheus, 2014

EDIT: Wilhelmina abdicated in 1948, but lived until 1962.