Answer:
[Español]:
La palabra ''alcalde'' es una palabra grave, y a pesar de que la silaba tónica está en la ''A'', la regla nos dice que palabra grave que termine en ''n, s o vocal (a, e, i, o, u) no se acentua o se pone tilde. Es por eso no se escribe con tilde.
[English]:
The word ''alcalde'' is a grave word, and although the tonic syllable is on the ''A'', the rule tells us that grave words ending in ''n, s or vowel (a, e, i, o, u) aren't accented. That is why it's not written with a tilde.
I hope I've helped you.