Why do the majority of Protestants celebrate Easter on the same day as the Catholic Church? There are likely some Protestants that either refuse to celebrate Easter or with Jehovah witnesses that celebrate on a different day to the Catholic Church. 

But the vast majority of Protestant ecclesial communities can be caught celebrating Easter at the same time as the Catholic Church. Every single year since 1518.
Protestants have not used the same date for Easter as Catholics for every year since 1518. They did until 1582, but in 1583 the Pope introduced the Gregorian Calendar. From then on Protestants generally calculated Easter in the traditional way, using the Julian Calendar, still used in most Orthodox churches.

The original principle related Easter to the Full Moon after the Equinox and ruled the equinox was March 21. By the sixteenth century the rule was out of kilter with the principle. From 1583 to 1752 Easter was usually celebrated by Protestants, in the British Empire, later than Catholics. In some parts of Germany a different method was used from 1700 and it survived in  Sweden into the 19th century.  It was astronomically based.

Sir Isaac Newton tried to introduce an astronomically fairly accurate method in England,  but eventually gave up saying the English would rather disagree with the sun and moon than agree with the pope. In 1752 the British Parliament decided to align the calendar generally with the Gregorian Calendar, though calling it the Georgian Calendar, and tables for Easter were produced on the Roman basis, though differently presented. This applied throughout the British Empire although it was binding only on the Church of England,  Church of Scotland and Church of Ireland, as regards religious observance.