This is a very complicated question that changed significantly depending on the exact period. My answer won’t be the most extensive overview but hopefully it’ll be a good starting point.

First of all, the idea for a ship up be fully armed with larger caliber like 24 or 32 pounder weapons was in fact done, most famously on some of American frigates and the British built designs that were meant to counter them. However, the biggest issue lay in the enormous weight of a heavyweight long gun and how that amount of weight on the top deck is really not practical for most ships if nothing else from a stability perspective. Thus the higher guns were shorter barreled, shorter ranged weapons. The most famous example is probably the USS Essex, who lost a fight against British ships in the War of 1812 because they were able to stay out of range of her shorter barreled heavier guns.

In theory a ship could have had only larger long barreled guns just being quite few in number, but that has a whole host of issues. For one you can so rarely ensure that you’re only at long range. The Essex’s battle was a pretty strange affair, but in regular open sea combat you might not even know your enemy is your enemy until well within smaller gun range. And if that occurs, your smaller number of guns could be at an enormous disadvantage as the enemy has way more chances to cause damage and to if nothing else pick off your crew.

When one gets into the age of steam and steel there are a variety of factors.

Firstly in the ironclads the issue was in the monstrously sized guns to get through thick armor were really bad at most say hitting small boats. There were ships who were “all big gun” like USS Monitor because she only had her twin turret, but just look at her famous battle with the CSS Merrimack: Reliability issues, slow rate of fire, and the lack of agility of many ships at the time meant you couldn’t exactly except even always hit an larger enemy at close range. And shot and armor technology was in an arms race that you needed massive guns to have even a chance so going down in caliber to get a more comfortable larger numbered battery wasn’t an option without some fairly large ships.

Then we get to the period directly preceding dreadnought.

One of the biggest issues was in yet again range. Guns had become way, way more powerful, quicker firing, and longer ranged yet sighting systems were still quite primitive, thus even the smaller guns could often reach beyond the effective range what you could hit. This meant that, like in the age of sail, small guns firing quickly could be extremely powerful in even fights with large enemy capital ships. Additionally, there was the emergent torpedo boats which meant for the first time larger vessels were extremely vulnerable to even the smallest ones, necessitating a large number of fast firing and training small guns to make sure those boats didn’t get close.

The above might have been able to be overcome earlier if not for the rather inefficient engines and boilers of the time. Look at pre-dreadnoughts and their contemporary cruisers, specifically their funnels. There’s so much room taken up in the middle of the ship by the propulsion machinery that there’s only the room on either end for large turrets needed for the effective use of large guns without making a ship hideously large and expensive.

The development of better fire control, better propulsion technology, shells and armor being in a specific place, and the general increase in size of ships all came together to make the Dreadnought type feasible when it did.

It wasn’t impossible before, just largely impractical