Why do helicopter operations avoid IFR?
Helicopters do not avoid IFR. They will, however, try to avoid IMC for safety reasons. The same is true for airplanes in the same weight class. Just because the pilot is IFR rated and the aircraft is IFR certified, it does not mean they will always fly IFR, whether they are fixed or rotary wing. Light aircraft in general don’t do well in extreme IMC, whether they are fixed or rotary wing. And, many of the same FARs in Parts 61, 91, 119, 135, 136 and 137 that would require an instrument rating apply almost the same to rotary-wing as they do to fixed-wing.
The main difference between fixed and rotary wing would be their environments of operation. Altitude would be a major factor. The majority of airspace below 1200-1500 feet AGL is uncontrolled airspace. You can not get an IFR clearance unless you are in controlled airspace. Since Part 91.119(d) allows helicopters to operate at reduced altitudes and distances from persons or property, they will specifically be used in applications not conducive to IFR flight.
If a helicopter were to fly IFR,they would be restricted to at least an altitude of 1,000 feet above the highest obstacle within a horizontal distance of 4 nautical miles from the course to be flown by Part 91.177. This would greatly reduce the usability of helicopters. The increased restrictions of IFR flight means that there is much more opportunity to make money in VFR flight with a reduction of risk.

§91.119 Minimum safe altitudes: General.
(d) Helicopters, powered parachutes, and weight-shift-control aircraft. If the operation is conducted without hazard to persons or property on the surface—
(1) A helicopter may be operated at less than the minimums prescribed in paragraph (b) or (c) of this section, provided each person operating the helicopter complies with any routes or altitudes specifically prescribed for helicopters by the FAA; and
(2) A powered parachute or weight-shift-control aircraft may be operated at less than the minimums prescribed in paragraph (c) of this section.

As far as your assertion that commercial fixed wing operations are almost exclusively IFR, that is not a completely true statement. While I will agree with you that may apply to Part 121 operations. That is not necessarily the case for all other operations. Many commercial operations depend exclusively on flying VFR.