The Royalists lost against the Parliamentarians because they were vastly outnumbered and also because of Cromwell's skill. Part of Cromwell's skill was that instead of attacking the cavalry from the front he attacked from behind Gorrinds cavalry. Then Bryson's pursuing of Manchester's troops was also foolish. Then they were outnumbering the Royalists by about 2 to 1.
The Battle
At 4am on July 2nd. Prince Rupert surprised the Parliamentarians by marching his army onto Marston Moor. He could of attacked before the parliamentarians were ready but waited for the Marquis of Newcastle troops to come from York. Newcastle's men were tired so arrived late in the afternoon. Rupert had dug and ditch and lined it with musketeers. By this time the Parliamentarians were ready and a surprise attack wasn't possible. Both sides put the cavalry on the flanks on either side of the ditch. The Royalists also put 2 groups of cavalry at the back centre. In the middle both sides put their foot soldiers. At 7pm the battle started.
The Battle at Marston Moor
In 1644 the Parliamentarian armies were besieging the city of York one of the Kings strongholds. On 14th June Charles sent a letter to his nephew, Prince Rupert asking him to help at York. Rupert marched into Yorkshire, when the Parliamentarian Commanders Thomas Forfix and Oliver Cromwell heard they moved their army onto Marston Moor, 4 miles west of York. So if Rupert wanted to rescue York he would have to fight them first. However Rupert surprised them by taking a 22 mile round trip, crossing 3 rivers to make camp just north of York. He ended the siege without a battle. He decided to fight the parliamentarians as he thought that the king had told him to do that as well. Even though he was vastly outnumbered and tired.