Here's why:
* Limited Unown Availability: HeartGold only features a select few Unown letters (U, B, L, A, C, K, and the "question mark" Unown).
* Specific Locations: These Unown are found in specific locations within the game, and K and O are not among them.
* No Breeding: You cannot breed Unown in HeartGold to obtain different letters.
To catch Unown K and O, you would need to play a different Pokemon game where they are available.
Please let me know if you have any other questions about Pokemon HeartGold.