there is no fundamental reason…
just its a bit simpler, and you use half the resources - and I guess, many have mono input sources.
to convert to stereo, at it simplest, you just need to duplicate the processing for the left and right channel, so that they are independent.
that’s the principle, but as you say, its dependent on the patch (and what you are trying to achieve) , so there is not one piece of code you can use.
its not a difficult task, so if your new to PD, its not a a good first project…
take one of the simpler samplers (preferably a C&G one), and give it a go, if you give it a try, then come back here with specific questions I’m sure you’ll get some help.
(tip: use a patch that only has a single screen of parameters, so you can avoid the complications caused in the UI by multiple pages of parameters)
EDIT: @Supermows, sorry, I meant to say it IS a good first project