Why not travelers' checks?

Just got back from vacation. Spring break in Tucson. Aboout half the places I shopped would not take travelers’ checks! Why the hell not? The only place they were universally weclome was Nogales, Mexico. What’s the problem with travelers checks?
They were American Express, if that makes a difference.

WAG: The places you went to didn’t take AMEX cards either, so they don’t have a merchant account set up with them?

You don’t need a merchant account to accept traveller’s checks, I think. I worked at a supermarket in high school and we accepted traveller’s checks but no credit cards at all.

They’ve fallen out of favor with travellers, so maybe the shopkeepers didn’t know what to do with them?

I worked at a bagel store, and although we didn’t take plastic, we did take travellers checks.

I think it’s a case of “Oh wow, what’s that? A traveller’s check? I’ve never seen 'em before. I’m not sure what they’re supposed to look like, or what I’m supposed to do with them, so I’ll just say we won’t take them.”