[nn3]

BCD is a requirement for many financial calculations, which these machines were targeted for (e.g. running payroll).

Since the hardware was extremely limited they could not afford to support both binary and decimal, so everything ended up being decimal. This was actually not an uncommon choice on some of the early computers. For example the ENIAC was decimal too, although that one wasn't designed for business.

There used to be a lot more experimentation in early computers before everything became standardized. The Russians even did some early ternary computers, there was a lot of variety in word length (36bit systems), the 8-bit byte wasn't even invented yet.