Why is it der Cottbuser?

Cottbuser 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] Residents of Cottbus, someone who comes from Cottbus

[1] Bewohner von Cottbus, jemand der aus Cottbus stammt

Examples

[1] My neighbor is a native of Cottbus

[1] Mein Wohnungsnachbar ist ein gebürtiger Cottbuser.

Declension

1 Singular Plural
Nominativ der Cottbuser die Cottbuser
Genitiv des Cottbusers der Cottbuser
Dativ dem Cottbuser den Cottbusern
Akkusativ den Cottbuser die Cottbuser

Dictionary entries include material from Wiktionary. Wiktionary