Let's talk about rifles a sec.

There seems to be a general belief in the "superiority" of rifles, and this imparts a number of axiomatic popular beliefs about 18th century conflicts, especially the War for Independence. And while it's true that rifles have a couple of advantages over smoothbore muskets, they also have a huge number of disadvantages.

The Pros

Pro: The rifle has a longer accurate range. A decent rifleman can easily hit targets at three or four times the range of a smoothbore musket, which is limited to accurate fire at 80 yards. Accuracy, in this case, means hitting a stationary, man-sized target somewhere on the body most of the time. A rifle can do that at 200 to maybe 300 yards, reliably. Pretty big accuracy bonus.

Pro: They're home-produced (at least the Pennsylvania long rifle and other derivatives). Most of the parts are fairly simple and they can be made by a cottage industry. Many men already have them.

Pro: We have exhausted the pros. Range and accuracy are really the big ones, but we'll come back to that later.

The Cons

Con: Rifles take a very long time to load properly. Maybe 30 seconds to a minute. The loading process involved not only the standard priming and muzzle-loading of a smoothbore, but because the barrel is rifled, it means that the bullet, to have any usefulness imparted by the rifling, needs to engage the rifling, meaning the bullet has to be a pretty tight fit in the barrel for the rifling to impart spin which will give the bullet its range and accuracy. For the most part, rifles were loaded with the bullet snugly encased in a greased cloth patch that acted as a lubricant to help load, but wasn't big enough to prevent the bullet from spinning. For the first shot, all this means is that the soldier has to use a little more umph in ramming.

The problem arises after that first shot. Black powder is dirty, and every shot leaves a thin residue called "fouling" that acts like quick-dry cement. It's wet and greasy when hot, but when it cools it turns thick and chalky, and it's hard to clean out. You can see where this is going: the fouling clogs up the grooves of the rifling and with every shot effectively makes the barrel aperture smaller, which makes it harder and harder to ram the next shot down. Your 30 second load time starts looking more like a minute, until you can't reliably load a shot at all, certainly not with a patched ball.

A musket, in skilled hands, can reload in 10-15 seconds, and a trained or experienced soldier can fire three reliably accurate, aimed shots a minute. It is pretty quick, relative to the expectations of the period. Furthermore, the smoothbore uses a deliberately undersized ball, relative to the barrel aperture - a .75 cal barrel on a Long or Short Land Pattern Service Musket (the Brown Besses) will generally fire a .72 or even a .69 caliber ball. This means that a soldier can reliably load even after the barrel gets gunked up by fouling.

Con: Civilian rifles can't mount bayonets. This is a critical shortcoming of rifles on the battlefield, because it means that rifle-armed soldiers are entirely vulnerable to bayonet charges, and especially cavalry. They need to be supported by nearby regulars or earthworks, and could not be relied on in close engagements. You could always give tomahawks or some other close-weapon for riflemen, but give me the choice and I'd take a bayonet every time. Plus, bayonet "fighting" never really amounted to much more than one side giving up and running away before the charge arrived where it intended. I've gone into more detail regarding bayonets in the period here.

Con: The rifles were produced by a cottage industry. They were not mass-produced, like the British muskets, and could not be purchased or produced in the numbers than an army would need.

Con: Rifles were more fragile than army muskets. They were made for civilians, so lightness was prized, but they were just not made for the kind of constant battering and exposure that military campaigns called for.

All in all, the rifle makes for a terrible battlefield weapon. It uses its usefulness as soon as the enemy gets within their own range, is completely vulnerable to charges by bayonet-armed infantry or cavalry, and takes a long time to load and fire. They make for great auxiliary troops, and having riflemen in enemy ranks can significantly alter the tactics employed by commanders.

The Long and Short Land Patterns by the British, as well as the Charleville model of French musket, were available in quantity, were reliable battlefield weapons, were rugged and dependable, and could mount bayonets.

But Americans did employ riflemen, but their usefulness is exaggerated much of the time. For now, it's probably enough to say that rifles were not the game-changer we like to think they were in the 18th century.

We can break this down to a few different components: cost and availability, usability, and fitness for the job.

Cost and availability: A 2017 version of your question could be "why isn't every fighter in the US Air Force an F-22?" The answer to both questions is the same: they are expensive, complicated, and difficult to produce, which makes them rare. It took a skilled gunsmith to achieve uniform rifling of the long barrel of the rifle, and few smiths in America had much practice in doing so. As such, the only people who had rifles tended to be those who made their living on accurate shooting: hunters and trappers in the backwoods. This community simply did not have the industrial capacity to support outfitting an entire army with rifles, and Congress did not have the money to pay for it.

Usability: Despite the modern myth that Revolutionary America was "A Nation of Riflemen," most Americans, and particularly the kind of men who joined the Continental Army, lacked extensive shooting experience. Continental soldiers tended to be younger and poorer than the American population at large, and a considerable number of them were African-Americans who may have been forbidden from owning a gun in civilian life. These were not men with extensive marksmanship experience. Putting soldiers through extensive marksmanship training to make the hassle and expense of a rifle worth it would have consumed a lot of time for an army that struggled greatly to retain soldiers (early-war enlistments were famously short).

Utility: Other than cost and availability, this was the biggest concern. In traditional 18th century warfare, accuracy actually doesn't count for that much. Instead, victory usually belonged to the side that put out the most lead. One solid volley of well-aimed rifle fire would not do as much damage to an enemy line of battle than the three or four volleys a unit armed with smooothbore muskets could put out in the same time. The grooves in the barrel that gave rifles their accuracy also meant that a shooter could not drop a ball straight down the barrel. Instead, the ball, wrapped in a greased patch of some sort, had to be hammered all the day down with a ramrod (wooden, as a metal one could damage the rifling). As such, loading a rifle took about a minute. That's great for shooting at deer, or when fighting from cover, but it is worse than useless when dealing with a charging battalion of grenadiers. The superior rate of fire of a smoothbore - one round every 15-20 seconds in the right hands - was preferable in most situations.

Not that melee combat was terribly common during the Revolution, but long rifles were too fragile to mount a bayonet. This meant that riflemen could not make the bayonet charge so central to infantry combat (to explain this seemingly contradicatory statement: charging with the bayonet happened with some frequency. Actually catching an enemy with the bayonet and coming to blows was rare).

Fortunately for us and unfortunately for the Americans involved, history did give us a direct clash between American riflemen and Brown Bess-carrying British light infantry: the Battle of Paoli. On the night of September 20, 1777, British "light bobs" pulled the flints from their muskets and charged a camp of American riflemen outside of Philadelphia using only their bayonets. After realizing they were under attack, the riflemen drew together and fired one volley at the advancing British. The volley was minimally effective - it was dark out, after all - and the Americans were torn to shreds before they could reload.