I think some people may not particularly value (or sometimes even completely grasp) whatever the accomplishment is, and I think, too, there are people who decide - if a person's getting enough compliments from enough people - that person doesn't need to hear yet more. Then, too, I think there are some people who figure the other person already knows he's accomplished something good and "there's no need to tell him since he already knows".
I'd guess, too, that there are probably people who think their own opinion/compliment either isn't worth much to the other person or else isn't worth much in general.
As someone else said, some people just generally have trouble expressing themselves, particularly if they need a certain type of setting/environment for one type of conversation or another.
Maybe, too, there are times when the person who accomplishes one thing or another interprets someone else's lack of offering a compliment as "...so hard for some people to give a compliment" simply because the person who accomplished something expects a compliment from everyone he runs into (or almost everyone) - essentially, expecting "everyone else" to make a bigger deal out of the accomplishment than - really - is called for.
OR, maybe the "compliment-withholder" is someone who, himself, feels uncomfortable with too much of a big deal being made out of things and he worries that the "accomplisher" is like he is - and so doesn't want to contribute to what may already be making "the accomplisher" too uncomfortable.
Then, too, I think there are people who make judgments about who deserves how many compliments/how much attention (whether that's an individual or group of individuals or a "type" of individual) "per any moment/time-interval" (etc.) and aren't (to say the least) people to risk "lavishing" such things on anyone deemed not to "need" "x-amount more" in terms of compliments.
Personally, I tend to think the "he already knows, so he doesn't need me to add to it and/or tell him with a compliment" may be one of the more common reasons some people lean toward being stingy with compliments. Not everyone is all that skilled when it comes to the "emotional end" of human nature - whether that's understanding those things or managing their own emotions.
Those are my guesses. :/