Why did losing Republican presidents in the past just meekly accept defeat instead of making baseless claims of voter fraud?

For one, it's not voter fraud.  State judicial branches usurped the legislative branches to change the voter laws.  That is what the lawsuits are about.  These states violated Article 2 of the Constitution.  All the other voter fraud cases, people have been sent to jail for it already.  There are thousands of those cases.

Personally, I don't like the idea of state courts being able to change laws whenever they feel like it, especially about voting laws (to make an insecure voting method MORE insecure: mail in ballots).  It's the job of the legislature to append or modify the laws, which is the proper process.  It would be like you going to court for reckless driving (speeding), then the judge says "Oh, by the way, I'm also going to sentence you for excessive reckless driving.  It's a new law I just made up to teach you a lesson".

Cause they understood that they could be voted out of office in 4 years

Because they were Adults.

They put country ahead of their own ambitions.

There is nothing typical about Donald Trump. He's not even a Republican. 

I'm reading the book written by Mary Trump - it makes a lot of sense now.