Obviously, I've been fed the answer that "the technology wasn't around before," but I'd like to know some specifics behind that technology. The Vikings seemed to have a coastline of ice to follow from Greenland to Newfoundland, but the Spanish-funded voyages that did it centuries later were on open water for weeks, so right now, I'm suspecting that the biggest "technology" in question was reliable celestial navigation developed from advanced astronomy developed by the Islamic caliphates during their golden age, and that the second-biggest would've been some kind of preservative or other other bit of nutritional chemistry that would've kept food lasting long enough for their voyage. I also figure that the financial state of the world's powers played a role too, that the monarchies of Spain/Portugal/France et. al. would not have had the money to fund these voyages during the Middle Ages.
Then again, since people were hyped up on religious dogma and tolerant with living short, unpredictable lives at the time I'm imagining that the manual labor to go on these voyages would've been far cheaper during the Middle Ages? And why do we never hear about any attempts from the Roman Empire to sail West out of (their colony of) Spain, when they already had a rough idea of the world's circumference and more stable coffers than the fiefdoms & monarchies of the middle ages did?
Did the unpredictability of there being *no* land West of Europe just spook sailors away from it that whole tme? Was there really no single, suicidal sailor who just stocked up six week's worth of food and pointed their boat in one direction before Columbus?
Instead of tackling the impossible task of explaining why nobody before attempted going west, I'll rather try to describe what was the setting of when Columbus did try, and it might help explain some things.
First thing to make clear, Columbus' attempt didn't come out of nowhere, like he was alone in the idea of sailing and discovering. While Columbus was slightly unusual (but not unique) to want to go West to reach East, in his time there was very much a very highly matured momentum of a sort, to go East.
It all started early in 15th century. You could take early French-Castilian expeditions/conquest of the Canaries as start, but as they were sporadic and disorganised, many place the start of the Discoveries with the Portuguese expeditions following their 1415 conquest of Ceuta. Reasons why and how Portuguese started their expeditions are complex, but to put it simply, their aims were local in nature. They wanted to go south to explore various commercial and military opportunities they could use to bolster themselves in their ongoing struggle against North African Moors. Soon they discovered that sailing south offered many chances of getting rich by trading with newly discovered peoples in gold dust, ivory, and most importantly slaves. They then proceeded further and further down Africa. By 1460 they reached Sierra Leone, in 1470s they went through entirety of Gulf of Guinea, and in 1480s they proceeded down modern Angola, reaching south Africa and passing Cape of Good Hope in 1488.
All this demonstrates that at the time there was a high amount of oceanic voyages and exploratory missions being conducted. The Iberians, Portuguese in particular, had found out that discovering new lands was profitable, and have set up an economic system that was economically sound and sustainable, even though regretfully the foundation for this economic profitability was trade in slaves. If you wanted me to identify the one thing that was different to all the times before, this would be it. Never before has there been so much activity in the Atlantic ocean, especially south of Iberia. This activity naturally lead to some highly important breakthroughs, one is the above mentioned confirmation that you could earn quite a money by exploring, but the others include maturing in the technology and skill of sailing, navigating and probably the crucial part: learning the Atlantic wind and current patterns.
To get back to Columbus in particular and his setting. The period most relevant to us, the 1480s, is the time of Portuguese king João II, and his quest to reach Indies. After exploiting the opportunities of Africa, Portuguese and João in particular realized the possibilities they could have if they reached Asia and cut out all the middle men that carry spices to Europe. João set up an organized push to reach the Indies and source of spices by organizing missions, hiring scientist and astronomers and developing navigational science (it is by their efforts that navigating by altitude of the sun was used for the first time). Columbus, who lived at Portugal islands of the time, was very much involved in sailing and trade with Africa, and was a part of wider circle around João II. And it is to João to whom Columbus first proposed the idea (which again, wasn't exactly his.). João II shot Columbus down, probably because he thought going around South Africa was a safer bet, so Columbus went to Portuguese rivals Spanish who - once their hands were free from finishing conquest of Granada - decided the gamble was worth it. And Columbus got his funding and was soon on his way.
To sum up and highlight the main points. Before Columbus, the initially slow forays south to Africa evolved into organized system of trade down Africa. Frequent voyages conducted by this systematic trade brought about both the technologies, skills and expertise needed for long sailing voyages in the Atlantic, as well as emboldened the Europeans that long voyages into unknown were both possible and profitable. Previously the idea of western passage was already formed, but in this setting it could finally be realized. Because you didn't really need one man, and some supplies, even for six weeks journey. You needed a crew of at least 20, with supplies for all of them, and a ship to hold them, and that was a major capital investment, one you don't gamble just like that.