1500 - 1750 represents 250 years, an exact quarter of the second millenium (ie from 1000 - 2000. Maybe that is part of the reason? If you are looking at history over the past 1000 or 2000 years, then breaking it down into 250 year periods could be useful in terms of looking at changes over a much wider period of time.
I would guess because it's a "long time ago" but from a period we still have plenty of documentary evidence about - eg. Tony Robinson unearthing a document that proves that Edward IV was illegitimate (although that's actually slightly earlier, as he died in 1482, I think)
well i think that it is most important because the 1500 was right near the start of the tudor times and these times ended at 1603 and also after that scotland was ruled by the king of england aswell which then was rare... the public had their own king beheaded.
i think that there are a lot of reasns why this period is so interesting and important. alot happened in these 250 years that kind of made the world we live in today.