to stop old code from breaking, i think it would be harsh to make it a requirement to use the new syntax exclusively immediately and break old code.
also when you very first start and are introducing the idea of digital input and don't want to introduce to many things at once.
Its useful for people to build the circuit with the external resistor so they can see the resistor - it provides a concrete demonstration of the two resistances that gives a swing in voltage on the pin (one changing resistance, the switch being an extreme example of this, one fixed resistance - this is a really important concept for many sensors, and what follows soon after, analogue sensors) and to add the internal resistor into the mix while doing that complicates unnecessarily at that point. Then introduce the idea that you can reduce the number of external components for the digital pins, but the trade off is the code gets slightly more complicated.