Going out of beta we had to decide on something which was really carrying what the app is, we loved Vector but it has always been a code-name and a nice pun on Matrix :)
Riot is more representative of what the app can do and its ambitions! Break the barriers between apps, give the control back to the user to choose their client, if they want to encrypt, host themselves, tune the notifs, the fact it's open, built on the open ecosystem of Matrix and thus benefiting of all the integrations and bridges built for Matrix.... Sounds pretty revolutionary to me ;)
Worth noting that only the name changed: the app and the team and the openness are still the same (modulo new features)!