The major reason for this was the decline of the feudal system. As feudal lords lost power, monarchs were able to take greater control of their territories and people.
The decline of feudalism came about for at least two main reasons. First, there was the increase in trade and in the economic importance of the middle class. This made cities more important and agricultural manors less important in economic terms. Second, there were changes in military technology. The rise of such things as longbows and even gunpowder made knights much less militarily valuable. Large armies were now much more important than individual knights. These two trends combined to make feudal lords much less important both militarily and economically. This allowed monarchs to keep greater control of their territory.