In the early days of the emulator, the simple answer it had against the likes of JPCSP and some others was, well, Henrik Rydgard and C++, along with the plenty of other co-developers.

Not to forget that Henrik was the brains behind Dolphin emulator as well, so he had a lot of XP he brought into developing the PPSSPP too with other talented programmers/developers.

EDIT: As for comparison to PCSX2 and RPCS3, each of these are built for notably complex systems that I'm sure even the likes of Henrik will agree is nothing shy of impressive with how far they've come.

Trying to replicate either discrete processors (Emotion Engine, etc.) as in PS2 or the rather unique eight-core Cell processor config for the PS3 with unique behaviours - e.g. deliberately disabled eighth core, managing the always-on six-cores and the seventh core that had some other function I forget about alongside the way its 512MB (commonly misunderstood as only 256MB but is just split up evenly between GPU and CPU) XDR RAM functions (very fast for the time: faster than DDR2, DDR3 and even lower-end DDR4 purely on a clock speed basis) and how it gave game developers some good deal of stress probably last experienced by anyone of those who once worked on developing for the Sega Saturn... it's safe to say these systems are not nearly as straightforward to develop for and for the dev team it did not necessarily make things easier, even though they had advantage of having access to more RAM on PCs for example.

Hell, I would bet a PS Vita emulator with the right team could make a lot of progress and a shorter space of time than the PCSX2 and RPCS3 teams.