The early modern period-- the end of the 16th and beginning of the 17th century is often referred to as a time of "military revolution", a term associated with Michael Roberts' 1955 lecture of the same name. While Geoffrey Parker and others have raised good points about some of the more exuberant claims, the core of the military revolution argument remains intact-- armies in 1750 looked very different than they did in 1600.

The most immediately obvious of these changes was that the pike had largely disappeared from the battlefield

Some reasons for this change spring to mind as most important:

1. A musket and pike formation has much less firepower than an all musket formation. The musket with bayonet is less capable than the pike, but it's adequate, and "adequate + more firepower" is better than "some pikes with a longer reach and less firepower"
2. The challenges of commanding a combined musket and pike formation like the Spanish tercio were very, very complex, requiring a high degree of skill and training. Getting musketeers to firing position was a challenge. The musket and pike formations were the province of skilled professional soldiers, but as armies got bigger and recruited less experienced men, less complex formations were favored.
3. Firearms steadily improved in quality, and armies got better at using them. Starting from the rather finicky, expensive and esoteric arquebus firepower evolved to much more easily manufactured, economical and reliable muskets: firepower got better, while pikes didn't. It was readily understood that firepower was increasing in capability, whereas polearms were little different from Alexander's sarissa
4. As firepower increased in efficacy, fewer battles were decided by melee. In the musket and pike formations, the notion had been that firepower was essentially an auxiliary force, a way of harassing and attriting the enemy, but that the decisive blow would be delivered by the collision of forces. 18th and 19th century forces did still come to blows hand to hand, but they typically fired many more rounds and did far more damage to the enemy by firepower before that happened. Many formations broke without ever having reached the enemy, or like Napoleon's Guard at Waterloo, were so bloodied and shaky from losses due to firepower that they had little shock effectiveness left when they finally reached the enemy.
5. As firepower improved, the threat from cavalry diminished. Horse are big targets, and by the end of the 18th century, cavalry was largely unable to break a well formed square-- in the Napoleonic wars, if memory serves, that happened only once, at the skirmish/battle of Garcia Hernandez in 1812. If bayonets and firepower were good enough to ward off cavalry, that was good enough; and they were most of the time. Cavalry had to surprise unprepared infantry, or run down broken formations to be effective-- against prepared infantry, they became progressively less effective in the 16th and 17th centuries.

Sources:

"Doctors of the Military Discipline": Technical Expertise and the Paradigm of the Spanish Soldier in the Early Modern Period

Tactical Evolution in the French Army, 1560-1660

Close Order and Close Quarter: The Culture of Combat in the West

Michael Roberts- The Military Revolution, 1560-1660

The "Military Revolution," 1560-1660--a Myth?