Otras lenguas romances como francés, portugués, catalán, etc., usan Ç para mostrar que una C se pronuncia /s/ y no como el sonido del latín viejo /k/. ¿Por qué el español rompe la tendencia?
Other Romance languages like French, Portuguese, Catalan, etc., use Ç to show that a C is pronounced /s/ and not like the old Latin sound /k/. Why is Spanish breaking with the trend?
More like: why did other Romance languages not drop the Ç, like Spanish did? ;)
Spanish has had a Ç for most of its history. It made its first appearances in texts from the 12th century and was still actively used well into the 1600s.
But during those five centuries, its original /ts/ pronunciation slowly shifted into /z/ (or /s/, depending on the region), and that's why Ç was officially excluded from the Spanish alphabet in 1741: because we already had Z, C and S to represent the sound Ç made by then.
Here you have the initial lines of a very well known book, taken from its first edition from 1605:
As @walen said in his answer, the ç letter did exist in Spanish but was rendered useless when its pronunciation was assimilated by other existing letters in the alphabet. But as you see it was well used in the 17th century. Nonetheless in the 18th century its use was pointless, as stated in the very first orthography book (PDF) by the Royal Spanish Academy from 1741:
In fact, the Ortografía says:
The use of the ç was the same of the z [...] and both are letter with the same pronunciation. [...] Since the year 1726 [the Academy] has been working in the six books of its dictionary [...] and in neither of its huge volumes the ç has been used, using the z in every case.
çis treated as a simplecin the search engine.