There are several possible explanations for why a planet rife with magic could be unable to move past the middle ages. One theory is that technological advancements have been replaced in this planet by the use of magic. If magic is easily accessible and gives many of the benefits that technology provides in our society, then there may not have been as much of a push to create other types of technology. If this is the case, then other forms of technology may have been less likely to have developed. This might result in a society in which people make use of magic to do things like light their homes, move commodities, and communicate with one another, but otherwise live in a manner that is comparable to how humans did in the middle ages.
The fact that magic is a limited resource is another another explanation for why a planet where it exists may be doomed to remain in the middle ages. If there is only a finite amount of magic in our world, then it is possible that only a small group of individuals are able to make use of it. 917
It's possible that these individuals are members of a strong wizarding class that restricts access to magic in order to preserve their power and retain their influence. It is possible that as a result of this, the great majority of people in this planet would be forced to live in abject poverty and fight tooth and nail for their own existence, while a select few members of an elite class will reap the advantages of magic.
There is also the possibility that the prevalence of magic has resulted in a standstill in the development of other aspects of society. If humans in this planet are able to address many of the challenges they face through the use of magic, then it's possible that they did not have as much of an incentive to develop in other areas. This might result in a world in which people live in societies that are like to those that existed in medieval Europe, but with the inclusion of magical elements.
There are, of course, a great number of other possible explanations for why a planet rife with magic may be frozen in the middle ages. In the end, the author or creator of the fictitious universe has complete control over the decision on the reason why this is the case. Regardless of the motivation behind the construction of the world, it is essential that it be logical and plausible. It may be difficult for readers or viewers to completely engage with the narrative if the reason why this universe is locked in medieval times is not well explained or does not make sense. 917