I make it clear in my cover letter that I am capable of picking up new technologies and languages with ease, which if you look at my resume and my personal projects on github is pretty plain and obvious -- I have 15+ years of professional experience in multiple languages and using all kinds of technologies. But they look at my current job/tech stack and say "hey, you don't match ours. sorry".
I even applied to dozens of jobs in the HN Who's Hiring, got a couple phone interviews, and nothing past that.
I finally got a job at a place that was using my stack and wants to convert it to something more modern.
This has definitely been a strange interviewing experience for me. I'm nearing 40 and have to wonder if ageism is becoming an issue for me.