IBM mainframes are big endian essentially because punch cards are big endian.
(Punch cards are big endian because the number 123 is punched as "123". So that's the order a decimal number will be stored in memory. The System/360 mainframes (1964) had a lot of support for decimal numbers and it would be kind of bizarre to store decimal numbers big-endian and binary numbers little-endian so everything was big endian. IBM's current mainframes are compatible with S/360.
On the other hand, in a serial computer, you operate on one bit at a time, so you need to start with the smallest bit for arithmetic. The Intel 8008 was a copy of a serial TTL computer, the Datapoint 2200, so it was little-endian. x86 is based on the 8008, so it kept the little-endian architecture.)
