When you try to take your spear with you all day, shopping in the marketplace, going in and out of buildings, and so on, you'll appreciate the advantages of a sword as an everyday-carry weapon. If you're a soldier, using a crossbow or an arquebus as your main weapon, you want a sidearm for close combat. If you try to bring along a spear as your sidearm, you'll find that it's difficult to carry both it and your crossbow/arquebus. If you're a soldier with a spear or pike as your main weapon, you want to carry a sidearm - it can be useful when spears/pikes get tangled up like https://commons.wikimedia.org/wiki/File:Bad-war.jpg

When swords were too expensive to be commonly used as sidearms, a large knife, or club/mace, or axe might be carried instead. A sword is a good sidearm, because it's long enough to have more reach than many other one-handed weapons, is easy to wear/carry all day, and effective enough in battle. When swords are cheap enough compared to the cost of armour, of pay for the soldiers, etc., then why shouldn't everybody have swords?

For more discussion on this, see my past answers in https://www.reddit.com/r/AskHistorians/comments/ae3b17/if_spears_were_generally_better_than_swords_why/ and https://www.reddit.com/r/AskHistorians/comments/ad4b1g/ive_recently_over_the_past_few_years_readviewed/

Hey there,

Just to let you know, your question is fine, and we're letting it stand. However, you should be aware that questions framed as 'Why didn't X do Y' relatively often don't get an answer that meets our standards (in our experience as moderators). There are a few reasons for this. Firstly, it often can be difficult to prove the counterfactual: historians know much more about what happened than what might have happened. Secondly, 'why didn't X do Y' questions are sometimes phrased in an ahistorical way. It's worth remembering that people in the past couldn't see into the future, and they generally didn't have all the information we now have about their situations; things that look obvious now didn't necessarily look that way at the time.

If you end up not getting a response after a day or two, consider asking a new question focusing instead on why what happened did happen (rather than why what didn't happen didn't happen) - this kind of question is more likely to get a response in our experience. Hope this helps!