Because of all the James Bond movies, the general public is familiar with the MI6 (collects Britain’s foreign intelligence). I don’t know why (It’s probably a secret) but they decided to rebrand the service and use the old name (but not the oldest): The Secret Intelligence Service (SIS).
A little bit of history:
Although the Special Branch was founded in 1883 and the Millitary Intelligence 5 in 1909 everything was so secret (Official Secrets Act 1911) that the service had no trace, no image and was just a rumor until 1991 when John Major first presented the British secret service and their boss at that time Stella Rimmington (surprisingly a woman just like in the James Bond movies 🙂 ) and officially in 1992 (the statute in the Intelligence Services Act was adopted in 1994).
The “M” name was there first: MI1(c) was born in 1914 when the first World War started. Now, although the official website now says that around 1920, it began increasingly to be referred to as the Secret Intelligence Service (SIS), the well-known professor and historian Cristian Troncota points out the fact that beginning with 1921 they changed the name to SIS (this is an interesting historic detail).
The thing is that between 1920-1992 the name was so well kept secret that the world believed in the MI6 brand. By 1994 they were out of the box but the world was still referring to them as MI6 and even they decided to play along and even have a MI6 logo.
Somewhere on the website they say that in the late 1930s the MI6 name was adopted as a flag of convenience for SIS and was the product of many writers and journalists. Nowadays it seems that they decided to take this matter into their own hands and promote a healthy new brand (the scandals with the Russians are just in the beginning).
I’ll say that with the new branding there might be a small issue: we have a the SIS in the UK and another one in the Republic of Moldova (of course, this is smaller and has no history – founded in 1999) 🙁