The easiest and most common explanation is that when Unix was created, an arbitrary time was needed in order to serve as the “start of time” for its core systems. This is referred to as the “Unix epoch” in Unix terms, and defines the earliest point at which a file can be created. The Unix epoch was sent as January 1, 1970.
So, simple answer, right? Actually, it’s a bit more complex. The truth is that Unix was not created in 1970, as there were rough versions of the Unix core around in the 1960s, and one of the earliest references to such an epoch was in the Unix Programmer’s Manual, which referenced it as “the time since 00:00:00, Jan. 1, 1971”. Yes, 1971 – not 1970. In theory, the epoch could be argued somewhere around 1969 when Ken Thompson and Dennis Ritchie started prototyping, but regardless, this still gives us an adequate epoch range of somewhere between 1969–1971
So why 1970? Well, according to Dennis Ritchie (who was one of the main engineers behind the Unix team at Bell Labs), it was set to 1970 because it was convenient to do so. I imagine setting everything to the simplest form – that is, 1/1/1970 as opposed to 1/1/1971, was just for simplicity and the resulting “simple” code.
