I suppose if it was deadly enough to take out stronger opponents, Piccolo would've used it more, at least in fights where he was willing to kill, like vs 3rd Form Freeza, Imperfect Cell, Android A17 or Semi-Perfect Cell. But he wouldn't have because it's too slow. He developed a quicker version, but maybe that version is put out with a lower power level. It usually takes more time to make a full power blast (eg a full power kamehameha). Makankosappo is also coiled, like a rope. It's quicker to make a rope with string that's easier to manipulate and it's quicker to make a rope that uses less material than a really a thick rope. So maybe it's quicker to make a weaker makankosappo. Yeh he got better at coiling a makankosappo so he can do the basic technique more quickly than before, but it's still too slow if putting in lots of ki.