Why is it der Ulmer?

Ulmer ends in -er, and most nouns ending in -er are masculine: in a German dictionary, 86% of them take der.

All nouns ending in -er
Sense 1
der

Meaning

[1] inhabitants of Ulm (male or indefinite sex)

[1] Einwohner von Ulm (männlich oder unbestimmten Geschlechts)

Examples

[1] Albert Einstein was born in Ulmerä

[1] Albert Einstein war gebürtiger Ulmer.

Declension

1 Singular Plural
Nominativ der Ulmer die Ulmer
Genitiv des Ulmers der Ulmer
Dativ dem Ulmer den Ulmern
Akkusativ den Ulmer die Ulmer

Dictionary entries include material from Wiktionary. Wiktionary