I think I figured it out. But first you have to know what '양성 모음(positive vowels)' and '음성 모음(negative vowels)' are, with a rule called 모음 조화(vowel harmony). Here are some informations from 국립국어원(National Institute of the Korean Language) which I briefly translated:
Positive vowels are the vowels giving 'bright and fresh' impressions. 'ㅏ', 'ㅗ', 'ㅑ', 'ㅛ', 'ㅘ', 'ㅚ', 'ㅐ' are them.
Negative vowels are the vowels giving 'dark and big' impressions. 'ㅓ', 'ㅜ', 'ㅕ', 'ㅠ', 'ㅔ', 'ㅝ', 'ㅟ', 'ㅖ' are them, and in most of the cases 'ㅡ' is also considered to be one of them.
Vowel harmony means a phenomenon which positive vowels like to 'hang out' with themselves, and same in the case of negative vowels: which means it is a bit rare to see positive vowels and negative vowels being right next to each other in a single word.
So let's go ahead and see whether this works or not.
1) ㅡ in 드 and ㅓ in 럽 are both negative.
2) ㅡ in 스 and ㅓ in 럽 are both negative.
3) ㅜ in 부 and ㅓ in 럽 are both negative.
4) ㅏ in 다 and ㅗ in 롭 are both positive.
5) ㅚ in 괴 and ㅗ in 롭 are both positive.
6) ㅚ in 외 and ㅗ in 롭 are both positive.
Problem solved. :)
I can understand most people saying 'there are no specific rules applied in here, just memorize it' because they are so used to it. Actually when I first saw your question I was almost the same. But since Hangeul is quite a scientifically sophisticated alphabetical structure, in most cases there is a rule which can explain something.