Twitter started as “an individual using an SMS service to communicate with a small group”, and SMS messages are 140 bytes (that may store 160 characters when using a 7-bit encoding such as https://en.wikipedia.org/wiki/GSM_03.38, but they didn’t go there)

I guess bumping the limit up while keeping the ability to interact with all kinds of SMS systems and ancient phones was a challenge, or just required waiting a few years for some of the weirder systems to die out.