Why is it sometimes hard to engage reverse gear in a manual transmission?

It's common for manual transmissions to not have the same gear syncronizing mechanism on reverse gear as it does on forward gears. The job of the synchronizer is not just to match the speeds of cogs that are about to mate, but to align the teeth so that they slide and interleave with each other. While reverse gears would ideally have the same alignment sync property they don't typically need the rotational speed sync mechanism because we (should/could) only shift into reverse when the vehicle is stationary

I suspect that you frequently manage to engage your reverse gears in such a way that the teeth bang into each other rather than interleaving as they should; perhaps you notice that when you "miss" reverse even though the stick looks like it's in the right "slot", it's not "fully back/forwards" compared to where it normally is. Moving to another gear causes a shift in the gear positions in the box and after this reverse is able to engage. Other actions, such as briefly raising the clutch or letting the car roll would also shift the cog positions, allowing the teeth to mate, though arguably the safest route is the one you currently take.

In terms of what's wrong, perhaps the previous owner played "if you can't find it, grind it" often with the reverse gear and has worn the teeth, that used to have meeting angled faces that assisted alignment, so that one or both now have flat spots; occasionally these flat spots collide directly, and cannot perform their function of causing cog rotation/alignment as they slide past each other:

Image credit:

Note that this image above makes most sense in combination with the following image:

The conical shape of the pink cog synchronizes the speeds, then it is the job of the angled faces on the two mating cogs to align the teeth. A reverse gear doesn't need the conical portion but it still needs angled teeth. Imagine what could happen some percentage of the time if the points of the angled teeth were worn flat so they engaged flat-to-flat rather than slope-to-slope:

It's an exaggerated image to make the point; teeth shaped with a flat will not engage if the flats meet (left diagram) but will if the slopes meet (right diagam), giving rise to a percentage risk of collide rather than slide. If your gear teeth happen to randomly be in the right place, reverse will engage. If in the wrong place, you have to do something to jiggle the gear positions