I changed because the GOP was voting against even things they supposedly agreed with, just to stick it to Obama. They were voting against helping veterans and against other things that I care about which they used to be interested in investing in -- such as infrastructure.
The Tea Party claimed to care about fiscal responsibility, and I helped elected some Tea Party candidates, but they turned out to be a Trojan Horse. After blocking anything Obama was doing, their other main interest was to pass legislation restricting abortion. I didn't vote for that. And in Scott Walker's case, he was moderate when we elected him, but then did a far right pivot in order to try to win the GOP nomination.
When the GOP was addressing fiscal matters during the Obama years, instead of doing anything meaningful there they did nickle and dime stuff. Like attacking food stamps, even though that was a very small part of the budget. And sometimes even doing things like pushing for drug testing of welfare recipients even if that would cost more than what they might save in welfare payments.
They had years to come up with a workable, humane "repeal and replace" proposition to give us instead of Obamacare, but we got crickets. When they finally got Trump for president and had a chance to pass good legislation, they had nothing. Because they had long ago stopped being about solutions. They were all about obstructing Obama. I saw this in 2014 when they got the Senate. I kept waiting for them to show they deserved the reins -- waiting for them to craft some common ground legislation which Obama could sign ... but they kept being do-nothings. So I gave up on them and started listening more to people like Warren and Sanders (and an acquaintance on another forum who was a somewhat progressive Republican and caught heck for it) and then the GOP went full-Trump and if I had any doubt, it was all gone. There was nothing left there for me in the GOP.
And when I cut the ties with the GOP, I was free to embrace some of my more liberal tendencies that I had been tamping down.