Archery is hard.
Like, really hard.
Fantasy almost never makes justice on how skilled and how strong an archer has to be to be effective in combat. While we often see archers as those lithe, quick, nimble elf-like beings with noodly arms that use bows because they lack upper body strength, that is the absolutely worst physical build possible if you want a good archer. You need those arms strong if you want to use a bow of old to any good skill level. It is you that is powering that arrow, not some magical fairy energy or something of the sort.
This happens because an ancient bow is, in essence, a very fancy spring. It takes the power from your upper body and shoves it into the arrow, sending it flying towards a target of your choice. Modern builds and techniques can help a quite a bit in decreasing the muscle mass you need to use those fancy springs effectively and for a long periods of time, but those aren't things you have in the Ye Old Times. Different bows have different needs, and old ones need muscles.
When you have a longbow, you have a very large, very fancy spring. A very large and very fancy spring that also needs very large muscles. Not hulk-large, mind you, but well-trained and well developed muscles. When you add the skill needed to fire a bow properly, you end up with a very exigent weapon that demands a lot of training, a lot of physical exercise, and a lot of patience to master. It is not a pick-up-and-use weapon like the sword or the spear. It isn't a weapon that you can hand out to your farmers and hope they will be useful in battle. Even if you give those bows to your best warriors, the chance of them having the skill and the correct muscle groups developed properly to use the weapon properly right away is slim to none. I'll elaborate on a few of the reasons of why that happens.
First, Longbows must fit their user. A large difference in height between two soldiers also means a difference in bow size. Give the bow of a very tall person to a very short person, and the very short person will struggle to make the weapon work properly.
Then, they need years of training. A sword or a spear is easy - you just wave the thing and it hurts people. You can train a lot to make yourself more effective at hurting people, but they are still simple weapons. A bow, on the other hand, needs specialized training. A longbow even more so. Those weapons are almost useless in the hands of newbies - they might even be able to fire a couple arrows, but those arrows will be inaccurate and weak. Heck, the newbie archer might even end up hurting themselves badly if they don't take proper care before letting the arrow go.
Finally, you need to know how to take proper care of the thing. Bows are finicky. Don't care of it properly, and you might end up with a broken bowstaff or a snapped bowstring on your hands. A dull sword is still a long, heavy stick that can be used to bonk people in the head. A snapped bow is no better than a walking cane.
Add all of that up, and you have a very hard-to-use and hard-to-master weapon that can be surprisingly effective in battle, but almost impossible to copy if you don't have years to spend training your people on how to use it.
So, make your Mystery Nation be a land full of mercenaries equipped with longbows and they will be a difficult force to deal with equipped with a weapon that, while isn't that hard to build, it is very frustrating to use.