Why do people call it a tabla mixta? Why not a tabla mezcla?
Tabla mixta is a food with two or more items, but it sounds wrong.
From the DRAE
- adj. Formado por varios elementos que se mezclan para componer otro. Apl. a cosa, u. t. c. s. m.
Check the entry for Mezcla as well.
Take into account that all the meanings of mezcla are nouns, but the one that matters for this case for mixta is an adjective.
When you are saying that that dish is a tabla mixta you express that is a "tabla" (a small board of wood to hold your food) and "mixta" is and adjective used to express that contains an assortment of things.
You could call it "tabla mezclada" but not "mezcla" because, as a noun, can't auqlify "tabla" the same way.
Others have clarified that an adjective made from the verb mezclar is actually mezclado -- in this case, the feminine, mezclada.
I will focus on when a Spanish learner should choose mixto versus mezclado, to express what in English is expressed with "mixed."
If you look in a usage collection resource such as linguee.com, you'll see many more examples of mixto, in contexts as varied as photographic techniques, economics, engineering, etc.
Here is a guide for choosing between mezclado and mixto:
Mezclado suggests a true blending into something homogeneous (for example La mezcla de ketchup y mayonesa se la conoce como "thousand island dressing" | The mixture of ketchup and mayonnaise is known as Thousand Island dressing), whereas mixto is more of a potpourri or combo. For example, restaurant menus in the US will offer a "combo platter" which has a combination of two or three different menu items. This is similar to your example, tabla mixta.
Novel combinations of techniques or approaches get described with mixto because they are a hybrid. If it helps, you might think of the Yiddish mishmash (where the last syllable is pronounced like the English word "mush," not like the English word "mash").
There is a special case, mixed race, which traditionally is expressed as either mestizo or mixto. This is a bit old-fashioned, in both languages. Nowadays, where you see mixed race or hapa in English, you might see mezcolanza in Spanish. Example:
"¿De dónde eres?"
"Soy una mezcolanza. Mi mamá es dominicana y mi papá es italiano; yo me crié principalmente en Nueva York."