In almost any airplane commercial operation, unless you’re doing something very short haul daytime ops, an Instrument Rating is all but required with airplanes(it’s even in the FARs). Generally the weather won’t stay nice enough for you not to use it, and most airlines and other commercial operations will avoid VFR all together, leaving VFR mostly for recreational pilots. In helicopters however it seems much different. Even in professional settings like news services, 135s Helicopter operations, and even essential services like fire rescue, will try and stay VFR at all costs, and will avoid flying if the weather is not VFR, even if they have an Instrument Rated helicopter pilot. VFR has a lot of limitations that would have a large chance of losing money for a helicopter service. Why is IFR so much more taboo and/or dangerous in helicopters compared to airplanes?
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.
Helicopter IFR operations do exist, but the short answer to the thrust of your question is: helicopter IFR is inherently more dangerous than fixed-wing IFR due to the lack of stability. A properly trimmed fixed-wing aircraft in good conditions could fly upwards of 30 seconds without pilot intervention, whereas a helicopter needs almost continual control inputs, thus leaving less margin for error.
To help overcome this risk, the certification requirements for helicopters are more onerous than fixed-wing aircraft, such as stability augmentation systems and autopilots. The effect of this historically has meant only the bigger and more expensive helis were actually IFR capable, so the pool of rotary pilots with IFR experience is much lower than fixed-wing pilots.
Finally, the obvious risks associated with helicopter operations - low to the ground, close to buildings, etc - do not work well with bad weather.
Here is a really interesting article which talks about these problems in depth.
There's another consideration: cost. Specifically, company overhead cost.
If you're flying commercially (let's focus on Part 135 to keep it simple), you cannot just "do an IFR flight" if/when A) the pilot is qualified and current, and B) the helicopter is certified and capable. Just because the FARs say you can, doesn't mean you can.
Every operator (no matter what Part) must have an Operations Specification (OpSpec), issued by the FAA. These OpSpecs will go into the minutia on what you can or cannot do, what airports you can or cannot use, if/when/how you can or cannot conduct an IFR flight, etc. (hundreds of pages of this stuff). These OpSpecs are then reviewed periodically, revised as needed, and "continuously monitored" by that operator's POI (Principal Operations Inspector) at the FAA.
So, back to the question (kind of). In order to conduct an IFR flight, it must be specifically stated in the OpSpecs that the company can do that IFR flight. In order for that authorization to make its way into the OpSpecs, the FAA must see that the company is maintaining a certain level of training for the pilots and a certain level of maintenance for the helicopters. These are expensive!! The costs of these mandatory requirements are not worth the benefit of being able to do the random IFR flight on those blue moon occasions when your point-to-point flight is going to be going through IMC and you need to get "into the system".
It's simply much more cost effective for most helicopter operators to say they just won't do IFR flights, and have that authorization withheld from their OpSpecs.