Reasons for having MCU pin-states default to pull-up/down out of reset

Because it's good practice to never leave logic pins purely floating.
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Other answers have given general reasons why a chip maker might make the choice to enable pull-ups by default. However, in the specific case of LPC845, there is an additional reason: it has specialized FAst Initialization Memory (FAIM) that can be used to set the state immediately after reset:

The FAIM contents provide a user-programmable initial configuration for aspects of the microcontroller, which take effect immediately after reset, before code begins to run. For instance, the standard I/O pads normally come out of reset with the internal pull-ups enabled. In some systems this may cause excess current to flow, until software can reconfigure the pads. However, by programming the FAIM appropriately, every pad's reset configuration can be customized.

(LPC84x user manual section 4.2)

Thus they've chosen the safe (from power usage and EMI point of view) default, while allowing more advanced users to customize the setting.

Back in the days there were Intel 8051 microcontrollers that only had open drain I/O pins, so most of the time you needed external pull-ups anyway to do useful things like connecting to pushbuttons or controlling CMOS inputs of other chips. This is most likely to have easy redesign of such boards with a modern microcontroller, or people from that era that are accustomed to designing with pulled-up open-collector I/Os. Back in the day, you mostly needed pull-ups if anything, and rarely pull-downs.