Because a) he lied about something, b) he did not have the resources, etc. to fight the charges, and c) they threatened to indict his son. So he figured the smartest and safest move was to plead guilty. He was probably right. Why did he lie? In all the fuss over the fake Russian controversy, he probably wanted to minimize his own involvement with the Russians. Not worth lying about, and nothing criminal happened (campaigns routinely have contact with foreign governments), but there it is. The fact that there is no underlying crime is sad but that’s the idiotic way our system works.
I think the government (and the press) create so much fear that even the innocent know they’ll be found guilty of something and are trying to protect themselves by any means. And then they get convicted for “lying” about something innocent because they’re so afraid.
I’m no big fan of Flynn, but at the same time, I think we should say that this whole process is worse than medieaval, and in fact is unAmerican and should be found to violate the Constitutional provisions intended to protect against Star Chambers and similar politically motivated proceedings.