Because it is seperated by need
Science requires that one goes with the evidence. That one looks at what happens, makes measurements, make predictions based upon those and checks to see if those happen.
If they do great but do it again and again
If it doesn't, well you we're wrong, your explanation of what happens was wrong.
Science seeks the how, the how things work, how things happen and happened. The how that can predict with allot of thinking and work of course what will happen next and.. be right
Black holes for instance we're theoretically known before they we're actually measured to exist. The calculations in mass in and gravity just.. had a hole. A black hole, well pun aside there was in the general calculations regarding masses in space a slight missing part. An HUGE amount of gravity keeping things together. And guess what .. they found it but only many years after it theoretical was known to probably exist.
Theories also can be wrong. And if such is found out such theories are put aside and changed where they are wrong.
Now lets to go to religion, oh yeah it doesn't works the same way does it.
Religion claims A, religion might make predictions upon A but such might happen.. or not. For everything a religion has claimed to happen at least something was claimed that Didn't Happen!
Religion Defines A, but.. offers no tools nor ways to test A. It says, lets take Christianity, Jesus is the son of God. But then.. how do you test such, how can you test such a claim.
You can't hence you can't use it to build upon.
The whole religious structure is build upon this system that A is as claimed without anybody speaking against it. And things are build around it, for instance that Jesus was born from a Virgin. Well how can you test that.. how can you be sure that is right. And then it even adds more on it by claiming that God gave 10 commandments. Hold a second, can you show us how... you can't
Hence in science if you claim something you have to show something for it. And if you can't yet hope that with allot of work a person in the future will indeed find out that you we're right. Until it is shown that there is a reason to support your idea it can't really be used.
In Religion you claim A and.. so it is. There is no test and we all have seen how people who ask to many questions we're treated.
Hence why, it simply are very different things.
Now if you are religious you always can say that science is the HOW things are done and religion is the WHY things are done.