Testing was done on version 1.19.5 on Minecraft 1.19.2

It's unituitively hard/expensive to enchant Straddleboards, since they can't get any low level enchantments, and anything that isn't Straddle Jump can only be found with very high level enchanting (from my testing I couldn't get any at 22 on the table, sometimes on 26, and got them regularly at 30).

Looking at the enchanting costs, Straddle Jump looks normal, you can get it starting from modified enchanting level 15, but the other 3 can't be obtained below modified enchanting level 30. Given the (i+1) in the formula, it sounds like a mistake. Since it can't get Unbreaking that explains why you can't enchant it at all at lower levels (since Straddle Jump I is the lowest you can get), and why you don't see the other 3 enchantments unless you almost max out the Enchanting Table.