Why did Parliament win the English Civil War?

The contrasting methods by which the respective sides raised funds was highly significant also, even more so as the war went on.  Parliament raised funds efficiently through taxes and tonnage and poundage from ports, whereas the Royalist's collection of funds was sporadic and very exhaustible- the King relied on gifts from loyal members of the gentry and aristocracy.  Most of the gentry and aristocracy remained0 loyal to the King; this not only supplied financial supplement, but also troops.  An example of one of these loyal royal supporters was the Marquis of Newcastle- he donated over £1million between 1642-1644 and supplied troops known as the 'Whitecoats'.  These donations were often substantial, however they did not represent a permanent source of income and the royalists were at times short of money, whereas Parliament’s financial position was relatively secure throughout.  Obviously this was a great advantage to Parliament, and cannot be ignored as a factor in Parliament's ultimate success.

Parliament went into the war with all the aforementioned advantages, however these were no substitute for the events that occurred during the war.  Throughout the war both sides' armies stubbornly remained in their locality for fear of their homes being plundered, with one notable exception; in 1643 London's trained bands left their posts to help their counterparts further north.  This was not a hugely significant factor but nevertheless it contributed to the Parliamentarian war effort.

By late 1643 another dilemma had mutually occurred among the respective sides.  This dilemma was a lack of troops.  The natural step was to seek the help of the Scots, and both sides attempted to do so.  The Scots however were opposed to the King after he had, a few years earlier, attempted to implement an Arminian prayer book into Scotland, this had led to war so the Scots sided with Parliament.  This signified a huge setback to the Royalists and a great achievement for Parliament.  Parliament also constructed the army of the Eastern Association.  This was significant not only for the extra troops it gave Parliament, but it also represented the emergence of Sir Thomas Fairfax and Oliver Cromwell.  These two new armies were augmentative to the Parliamentarian war-effort and undermined the Royalist's endeavours.

During the winter of 1644 perhaps the most significant factors in Parliament's ultimate success occurred.  These were the introduction of the New Modal Army and the passing of the Self Denying Ordinance.  The former meant the integration of all Parliament's respective armies into one single, unified field army with a clear leadership structure under Cromwell and Fairfax.  Before this reformation Parliament had had no real leader, this had been manifested in a lack of co-operation and organisation among the Parliamentarians.  The latter of the two reforms prohibited any MP from holding a command in the New Modal Army (with the exception of Cromwell).  This was significant as it separated politics from fighting; a combination that had led to defeat for Parliament in the battle of Newbury when the Earl of Manchester had had a separate political agenda.  

Cromwell's rise to prominence was key in the Parliamentarian victory.  The Parliamentarian army was thereafter a more decisive unit, this is especially significant when compared to that of the Royalist's who, although they had one main leader in Charles, were highly indecisive; perhaps a reflection of their leader's personality.  Cromwell had an ability to rally cavalry and use it with previously unparalleled success against the enemy's cavalry.  Cromwell and Fairfax's influence was especially evident in the battle of Marston Moor.  This battle in 1644 signalled the end of the King's northern stronghold and was therefore a significant factor in Parliament's eventual success.  A yet more decisive battle came in June 1645 on the grounds of Naseby.  Here came the definite realisation of the Parliamentary victory to come when, the Royalist's last main field army was beaten by Cromwell, Fairfax and their many pawns.  Then came the final major battle of the English Civil War; the battle of Longport- a mere formality for the Parliamentarians, its only significance derived from the perspective of hindsight when its finality is demonstrated.  This was effectively the end of the war.  The aforementioned battles laid the foundations for and signalled the end of The English Civil War and Parliament's consequent escalation to sole governors of England and her lands.

In conclusion, it is not possible to state one reason or event that, beyond all others, was responsible for Parliament’s victorious Civil War campaign.  A whole plethora of factors were responsible for Parliament’s feat and it was the amalgamation of these factors that led to their victory.