Probably because the PDP & Intel 8080 were typically octal systems rather than hex, the 8080 instruction set decoded nicely in octal (or at least the MOV A,H instructions). By the time the 8086 arrived the micro-computer world had moved to hex as it nice decode mapping was broken by the Z80 and the 6502/68xx families didn't have that nice split.
So my guess this is to allow 12 octal to be 10 decimal as that would have been the default in the inital code base.
I thought the whole octal business stemmed from the time mainframes and minis used 36b, 18b words respectively as 6bit (early mainframe character width) can be conveniently represented as two digit octal. This changed when computers went 16/32bit and started to use 8bit bytes (and ASCII encoding of characters).
So my guess this is to allow 12 octal to be 10 decimal as that would have been the default in the inital code base.