Hi, good question. I'm not sure how to answer though :) I need to get Timothy involved for that (copied here) as he was the one who promoted the change.
Honestly, I was happy with pd-lib-builder, but things got complex and the it was I was just not able to build it all at once and had to do it separately for some special objects, namely circuit~, sfz~, sfont~, pdlink/pdlink~, play.file~ and sfload (and maybe something 'else').
When asking for Tim's help to simplify the build system he was just more comfortable in using this other system instead as he was more familiar with it, so it seems.
I could go back to pdlibbuilder any time I guess, but I would need help in order to make it all more streamlined.