I guess may be why it got so resistant to so much is that doctors have ignored blastocystis since the introduction of Antibiotics .
10% to 20%, may be 30% of western population are carriers and every time person get treated for something else the blasto get chance to adapt to drugs.
In 2006 research showed that 11% of US population likely are carriers of blasto.
Many different sub-spieces could poosibly explain why some get much more sick from it, genetics could play a role, how long it has been attacking etc etc.