umm if you open up to the preface of bukhari and Muslim their biography including how they collected the hadith is explained... they are classified to be the most authentic coz of how vigorous they where with regards to validating and authenticating whatever they where told.

Other scholars of hadith collected hadith but wrote down and recorded the weak ones as well (fromt he point of view that it's better to not hide knowledge, and so it was important to write everythign down btu make note of weak or false narrations etc.), some of the other scholars where also more lax in their method of authenticating hadith

which left teh two Sahih's as the two sources that stood out from all the others..

no not really, they're there for all times, especially if you look at the hadiths that pertain to the end of time, and the hadiths where the prophet is giving advises to those that come after him, they are directed at Todays Muslims more than those of previous times...

there are also many rulings that can not be derived fromt he quran, for example, the way Muslims ought to pray is not described in the Quran, but it's descriebd in great detail in the hadith, similarly the punishment for some crimes is detailed in the Hadith but not in the Quran and many other things..