I haven't fully investigated this yet, but I have ran into old news paper articles from the 20's an 30's (earlier?) that seemed to suggest that the swastika became a fad to virtual signal your support of socialism. Even here in the US. I'll have to see if I can dig it back up.

So it makes sense to me that some old European countries held on to it.

The left wants you to forget, or never know, that THEY spawned the nationalist socialist party fuckwits. It was essentially their pride symbol of that era. Not for gays, because they had no political use for them at the time, but for their political movement. And make no mistake, the pride flag today is really about the left and little to do with one's bedroom proclivities.