The problem is that each person has different needs and it's difficult to make a site that is usable for most of them - that's why so many 'niche' sites exist.
For example I used to fly a lot and most of the time I did not care about exact dates at all. I guess that scenarios like 'I live in Spain now and I want to visit my parents in the Czech Republic some time in June' or 'I want to spend some weekend in Paris' are pretty common.
Sometimes I don't even care about destination that much - I want to travel somewhere in south of Europe to take a couple of days off - all I care about is the price. Date-of-flight-centric sites are really difficult to use for me. Only recently I discovered http://skypicker.com (I'm not affiliated wit them) that does what I want - I want to fly to Spain some time in August - here are cheapest flights.
For example I used to fly a lot and most of the time I did not care about exact dates at all. I guess that scenarios like 'I live in Spain now and I want to visit my parents in the Czech Republic some time in June' or 'I want to spend some weekend in Paris' are pretty common.
Sometimes I don't even care about destination that much - I want to travel somewhere in south of Europe to take a couple of days off - all I care about is the price. Date-of-flight-centric sites are really difficult to use for me. Only recently I discovered http://skypicker.com (I'm not affiliated wit them) that does what I want - I want to fly to Spain some time in August - here are cheapest flights.