The Borg also have advanced medical technology, death to Federation doctors, may be near death to Borg medical technology. For example the cryonically frozen people discovered in the episode, The Neutral Zone
, were dead to 20th century doctors, but not to Dr. Crusher. To the Borg, the few seconds between the taking of the pill and the nonoprobes injection may be more than enough time to prevent the killer pill to take effect.
What then happens when a Starfleet ship comes across a Borg ship in open space where no Borg activity had been detected? Have some run and get the suicide pills while the starship makes a brake for it?
And I will ask you to look at it from the standpoint of the Enterprise crew in Best of Both Worlds
. They didn't know about assimilation and they didn't know the repercussions of assimilation. They knew the Borg wanted the Captain, but not for what reason. Extra security would mean more phaser fire on the bridge. And as was pointed out Picard was taken off guard, surprised, even with a suicide pill he may not have had time to activate it and maybe the Borg had already neutralized it.
As for Starfleet at Wolf 359 I think it's fairly obvious that no suicide pills were given to the command crews. Otherwise Sisko's job, after the Saratoga was mortally wounded, would have been bite the pill before the Borg beam on board, but instead he went lookingfor his family and exacuating civilians. If these suicide pills you suggest were indeed given to the command crew as you also sugested, didn't Sisko show dereliction of duty (sorry about mispellings not on my regular computer) by not taking his?
We can play arm-chair-quarterback all we want (and we do a lot of that here for situations that never happened) but our heroes didn't know what they were in for.