Because the OP didn't ask about STR dagger.
Out of those options yes STR dagger would give the biggest boost, but you can't forget about both WSacc and Primary mods that the dex dagger gives. Also, you're not gonna have capped acc on everything, so for a situational dagger it's a very nice boost. Of course the best option would be to get both and use them accordingly. I myself use str and dex +1 on most higher level NM mobs that have crazy evasion....everything else it's str and oa3 para +1 (unless I'm tanking, then it's double agi daggers of course).
I understand that he was asking between the DEX and AGI daggers, but suggesting STR also along with an answer to his question wouldn't be out of line.
Honestly, with how situational this game is, and DNC especially since we can fill so many roles, there wasn't enough information in the OP to really tell him exactly what to get. If he does a lot of soloing on DNC and uses other jobs for group play, AGI daggers would benefit him more since anything he's going to be soloing, he's generally going to have decent accuracy on. If he's suited more to a main healer role on very high end and specific mobs (most normal EXP mobs anymore aren't very evasive, and if you're missing it's more of a skill/level thing than gear), DEX daggers would be more beneficial.
My comment was that if he plays more like I do, focusing on more of a supporting DD role, a STR dagger would be most beneficial. I'm not sure how long the OP has been around, but if he's a newer player he may not fully understand how damage and evasion is calculated and may not really understand what stats would be most beneficial, hence why he only asked about AGI/DEX daggers. TBH, depending on what you're fighting, PDT- daggers and MDB+ daggers both have a place in a career DNC's arsenal, as both would have their situational uses, so why not add those into the mix?
As for the WS mod and WS acc, both STR and attack factor into WS damage, and most WS gear already has a bunch of acc on it. Personally, I have more acc in my WS set than my TP set, and I'm near enough to cap in my TP phase on all but the most evasive mobs that I have long since stopped considering DEX daggers, since it takes *far* less work to just swap in a full accuracy set and pop a pizza than work through a thunder elemental path.
Also, how the heck did you stay sane going through oa2-3 para+1...I'm halfway through piestes and want to pull my hair out. -.-;