3.3V is the lowest voltage rail so the drops on it bring bigger deviations compared to 12V and 5V.
Also, very briefly explained, all modern PSUs use DC-DC converters which in most cases (as it seems) cannot deliver tight voltage regulation. On top of that even in PSUs where DC-DC converters are not used still 3.3V are not truly independent generated (except in some very few cases) so high loads at this rail bring equally high drops.
Finally don't forget the fact that since all modern systems draw mostly from +12V and the minor rails are not heavily used (because of that they called minor) the manufacturers focus at +12V and not at 5V and 3.3V.
Forgot to mention that although 5V are drawn from 4 pin peripheral Molex connectors besides the main ATX (5 wires), 3.3V are exclusive drawn from the main ATX - 4 wires- (ok and from SATA connectors but you can't drawn significant power from SATA compared to 4 pin).