Because science and tech journalism requires a certain knowledge of science and technology, but people who want to go into journalism generally don't study those topics and aren't interested in them.

Science journalists often do have a scientific background, like Ed Yong for example (MPhil Biochemistry).

There are some great ones out there, particularly at ArsTechnica, but there are also journalists working at mainstream newspapers who are sent/told to cover some tech/science thing and presumably they do their best but they end up out of their depth and just put something together to meet the deadline.

^ This.

My wife was a high level reporter in DC covering the White House and even traveling with POTUS at times (locally via motorcade, never on Air Force One). I can't count the number of times she'd call and say "I have a 3pm deadline.. what do you know about X?"

The number of reporters who have special training or specific background within a beat are exceptionally small. They're there but they are NOT the norm.