The grass streak is most impressive since it's such a short patch of the year, it's harder to master it and completely own the competition.
A short season means that you don't have to maintain form for long. A longer season requires consistency.
It also means you need more years to do a big streak than you do in the other surfaces, which also makes it harder to accomplish.
So winning the U.S. Open 4 times in a row is a greater achievement than the Calendar Year Grand Slam?
Just playing devil's advocate