Sadaqat al-fitr has to be given in Ramadan because it is a worship related to fasting. A hadith from our Prophet (p.b.u.h.) is as follows:
“Whoever gives sadaqah-al fitr before Eid prayer it is valid zakat, and whoever gives it after the prayer it is just a kind of sadaqah.” (Bukhari, Zakat, 1503)
Zakat on the other hand is an annual worship. Possessions of money, gold, commodities, animals etc. are subject to zakat on a yearly basis. Therefore, zakat is not a worship pertaining to Ramadan only. It is given outside of Ramadan, too.