Bits? We've got lots of bits, but how many do you really need?
"Seriously, in this day and age?
Stop messing about and use 64- or 128-bit for everything"
But GPS originated in the 1970s, and also has the constraint of needing to transmit data from around 20,200 km altitude to down here using not very powerful transmitters. That means there's an engineering need to minimise the amount of data transmitted, and also one must consider backwards compatibility.
Going up to 13 bits from 10 bits in this case makes sense: it extends the week counter from around 20 years life to 8 times that, i.e., around 160 years, by which time I dare say the current GPS system of operation will be long gone. Yes, you could make it 128 bit and that would last long beyond the heat death of not just this universe, but some beyond (although I suspect relativistic effects involved in the collapse of one universe followed by a subsequent big bang might well invalidate all assumptions regarding the passage of time). I'm not convinced that's entirely useful.