The other great answers people already gave aside, there's some things that weren't mentioned: the fact that until the modern era there wasn't a notion of unified country as there is today.

In a sense, during the middle ages, there was no England or France, rather smaller duchies, princedoms, counties and so on, unified by a cultural identity in common (speaking the same language, sharing ancestry bonds, serving under the same dinasty, etc). This means there was no ambition of "conquering all of France" by the english or vice-versa, since neither viewed the other as one unified political and territorial body.

There were also more pragmatic reasons for an attempt at total conquest not being interesting (or even feasible). For the english to take all the frankish smaller states, for instance, it would mean waging war against many factions other than just against the french king and those who rallied to his call. Read the comment by /u/Scrappy_doo_07, there's no need to explain it any more than that.

Also, think those were just the difficulties for the conquest, but there was also the matter of keeping conquered territory. The english learned this when they lost control over brittany, and with it the foothold they had in the european mainland, as well as the undisputed control over the English Channel.

These are just some of the factors that didn't allow for a total conquest of the Frenh over the English or vice-versa. Together wth the other comments, I think everything has been very well explored.

Edited to remove some weird text the autocorrect must have inserted by accident (and I totallly didn't see when proofreading).

• Fighting wars are expensive. The kings of France and England spent most of this period raising money than spending it until they are almost bankrupt. They could only afford to keep armies in the field for a small period of time and conquering a kingdom takes time.

• Castles and walled cities take time to capture. During this time a relief army would often show up or the besiegers would lose a lot of their men to disease and have to fall back.

• It was more profitable for the nobility of England to raid rather than Conquer. The battle of Poitiers which was a major victory for the English under the Black Prince happened because the Prince was raiding and looting the country side of France and got caught out in the open by French forces. The Black Prince wasn't campaigning to take region but to fill his pocket book and to reign terror in the hearts of the French. As one can imagine this tactic used by the English during the war made it hard for them to gain total victory.

• The important leaders would often die at inconvenient times. For example Henry the 5th was named heir to French throne and regent of France and then died before he could take advantage of all that he won leaving a boy as the King England.

In summary the only way to conquer a country is to actually hold large castles and towns. It takes time for a besieging army to capture said strongholds. To keep armies in the field to capture these strongholds it takes a lot of money which is not always easy for a king to raise. One way that the English raised this money was by raiding the countryside of France. Think Shermans march to the seas but through France every summer. As these raides become more profitable the English Nobles much prefered raiding and looting to siege warfare and raiders conquers don't make. So you could say the reason the English never finished off the French was because they would rather raid all summer then take of important Towns. The main reason the French never finished off the English is because every summer the English burned and looted their countryside destroying the economy which makes it hard to raise the armies needed to finish off the English. It is also important to keep in mind what is good for the King and what is good for the nobles are not always the same thing.

The war was ended by the French do to the invention of the cannon which made it much easier to take walled cities and castles. This met in a campaign season the French could take back 10 times more territory than previous years.

What I remember from reading Desmond Seward's excellent book "The Hundred Years War"