I can chime in concerning the Stuart Crown, which ruled over Britain during the 17th century. During this period, the monarch not only lost certain powers and privileges, but was periodically abolished altogether. It is an incredibly complex period, and subject to much revision, in which political, religious, economic and social disagreements between King and Parliament eventually led to civil war and execution of Charles I. However, I will try to focus on the on the conceptual conflict behind this period, as an explanation to your question.

"Why" Kings and Queens lost" their power is a great question. In the case of the Stuarts (James I, Charles I, James II and Charles II), it was certainly not a deliberate change on behalf of the monarchs. It is also worth noting that they were not "all-powerful." Certainly, the Stuarts retained prerogative powers during their rule, which theoretically allowed them to rule without impunity. However, in practice, this was not the case. Parliament maintained its own authority as a constitutional body, and was not afraid to question royal authority.

I think it is important to understand how and why Kings and Queens ruled with absolute authority. To summarize briefly, the King ruled by divine authority, which is supreme above all institutional powers and laws. So, he can, by that logic, carry out policies and laws without earthly consequence. After all, he is only answerable to God. However, the idea that he rules by divine authority becomes complex, and contentious for that matter, in conjunction with Parliamentary institution. Indeed, how can a King retain absolute power while having to remain within the confines of civil law? As such, the Stuart Kings largely operated in a grey area concerning their absolute power. For example, James I tried and executed Guy Fawkes by using the Star Chamber, an arbitrary court which operated under royal prerogative. However, given the context of the offense, trying to blow up Parliament, no one questioned his authority in doing so. Conversely, when Charles I used the same court to controversially mutilate dissenters as a punishment for printing 'seditious' works, he was met with stark opposition within Parliament which eventually led to the abolition of the Star Chamber itself. Yet, if we examine the two cases, both were perfectly within their right, under royal prerogative, to employ the Star Chamber. As such, a key part of how the Stuart Monarchy lost certain privileges and eventually collapsed is that, when examined in depth, the relationship between an absolute monarchy and a constitutional parliament is problematic.

Thus, to answer your question in part, a couple of hundred years ago, the Stuart Monarch was not "all-powerful" in practice. In fact, a great deal of tact was needed to employ the presumed absolute power of monarchy without butting heads with Parliament, as the above example illustrates. Charles I made the fatal mistake of presuming and acting as if he had absolute authority, which in short, led to a civil war and his eventual execution. This is a broad, and incredibly in-depth question, so I would happily answer further questions concerning the Stuart Crown. For further readings, I would consult works by Tim Harris, particularly Rebellion. Additionally, Woolrych, Britain in Revolution, is another key work and incredibly in-depth. Other authors that I highly recommend are Mark Kishlansky and David L Smith.