In the original games, the Paladins had the power armor, and were the elite soldiers/officers. Knights were like enlisted grunts and non-comms who got to do all the grunt work and work in the "motorpool", and if they went soldiering, it was in standard combat armor.

Since the Bethesda games, that's been changing, until Fallout 4's rendition, where Scribes now handle all of the tech work, Knights are soldiers, some with combat armor and some with power armor, and Paladins seemingly are even more elite and rare.

We can probably blame this one the newer Brotherhood's seemingly larger supplies of T-60 power armor (however it is they got them), and losing a lot of the more experienced soldiers during campaigns against the super mutants and Enclave. So now that power armor is more plentiful, and veterans more rare, the bar for who gets it is lowered, which also makes their general forces a lot more dangerous, and with more of the Knights in a field role, Scribes have had to take up the tech slack.