And destitute serfs, as an estate, had very little political clout; whereas the Swiss turned around and marched home the minute they weren’t paid (Pas de’argent, pas de Suisse) and the burghers often refused to march more than a couple of nights journey from town or for more than a few days. If they were pressured too much they might revolt.
So for the prince, instead of a small ‘bespoke’ army with a small number of highly skilled but expensive fighters, it was very tempting to go instead for a much larger army of lesser paid, and (at least initially) less skilled peasants or serfs, but who were far more biddable and didn’t need to be paid nearly as much. And all you really needed them to do was either stand there with a pike or shoot a musket volley when told too.
The Ottomans and Mamluks of course took this a step further by using slave-soldiers.