Just wondered. What caused the newspaper adventure strips to die out? Was it comic books becoming more popular, newspaper syndicators closing or just lack of interest?

I know I never read them much as a kid despite reading newspaper strips in general. I always read the funny comic strips but rarely bothered with the longer serialized adventure strips in that format.

Their readers eventually died off.

Speaking of readers dying off, is Gasoline Alley still around?

GoComics.com has it. The copyright is 2016, but I'm not sure if that's edited in or if it's actually still running. Here's a link

In modern terms (last twenty+ years), size has been the biggest factor. In earlier decades I would guess TV and silver age comic books probably added to the general atmosphere that made them less and less of the compulsive habit that they were for audiences in the 30's-40's and into the 50's.
They co-existed quite enthusiastically with Radio as the primary mass media. But each was providing something the other lacked.
And then how excited could you get for a continuing 4-5 panels of content each day when storytelling was becoming more sophisticated and the content was 20 pages of continuity in one go, as happened in the Silver age?

It's an interesting question, since funny strips are still alive. I guess it's just because other media developed that were better for telling those kinds of stories, like comic books or TV.

I'd like to think it's the fact that they move at a snail's pace. I wasn't around for the heyday, but holy shit, Spider-Man took 6 months to tell a simple story.

I used to cut them out and put them together like a comic book. Sundays were frustrating with the larger size, and often two I was collecting would be back to back on the page and couldn't get them both.

A quarter would have bought you a second copy of the paper!

On a Sunday? Maybe in 1970.

Though I remember when it was a quarter for a daily Washington Post 15 years ago. Now it's like quintuple the price.

I think these two posts hit the nail on the head. TV and comic books do a much better job of telling a story to people who have no patience. I could never get into adventure strips as a kid. I'm not entirely sure I could do so as an adult either. The only reason I can read the old ones is that I can buy a book with 2-3 years of strips and read them within a few hours.