I don't have any great knowledge of the history of Germany after 1918, but I believe that most at least of the royal families remained fairly popular and were treated with respect. Perhaps one could look at it like this. The monarchies were swept away in the wave of disorder that followed the defeat of Germany. If that had not happened and votes had been held on the matter after the situation had become more stable, I imagine that the will would not have been there to get rid of them. (Someone may be able to say something more definite about this). But once a monarchy has been abolished it is quite difficult to re-establish because, instead of it just being there with all its appurtenances from time immemorial, a consensus has to be reached not only to restore it, but to reach all kinds of decisions about its powers, incomes, properties etc. This raises such problems that it is much easier even for those who are broadly sympathetic to the monarchy to do nothing and let sleeping dogs lie. Furthermore, opinions inevitably become divided according to party lines. The left tends to have no love for monarchies if not to be postively republican, while the active pressure for restoration tends to come from the right (often fairly far right in conditions like those in Germany following the war, when opinions were so polarized). This, I think, is really fatal for any monarchy because, instead of being there by tradition, it is seen as representing a factional interest, and becomes an object of factional dispute. There was the further problem in Germany of the position of the Hohenzollerns. In view of the part that the Kaiser had played in German politics, it is understandable that the Social Democrats in particular would have had no wish to see another Emperor. It was not as if a constitutional and limited monarchy on the British model had been removed. Some of the smaller monarchies had still been fairly feudal in nature moreover. So there seem to have been quite a number of reasons why there was no political pressure to restore the monarchies even if the monarchies themselves may not have been unpopular.
[When talking of left-right divisions regarding monarchies, I was thinking of people who are actively engaged in politics. In my own country, the UK, for instance, left-leaning politicians tend to have less sympathy for the monarchy than right-leaning ones, but it is by no means the case that people who vote on the left are predominantly unsympathetic to the monarchy. As a consequence, the matter has never really become a party issue. But if one can imagine that the British monarchy had been swept away in a coup or something in the 1980's and those same politicians had to decide whether or not to restore it, the matter would surely then become a party issue that would tend to divide on left-right lines. That does not mean that a monarchy cannot credibly present itself as standing above the pollitical fray as long as it remains stable. And monarchies in Europe do seem to be remarkably stable as long as disorder does not arise as a result of failure in war etc. ]