Most higher profile musical acts will have a contract to perform. The contract is designed to spell out clearly all the relevant details regarding the performance [i.e. compensation, merchandise arrangements, etc.]
Attached to this contract will generally be a 'Tech rider'. I have no idea where the legal terminology came from, but i suspect it's basically what it sounds like. This is something that is 'riding along' with the contract and spells out technical requirements and the like.
When one speaks of gear being 'rider friendly' it simply means that it is equipment that is likely to be acceptable to meet these tech rider requirements. Often specific mixing consoles or speaker systems will be listed in the rider. Typically there will be several acceptable options listed. 'Rider friendly' options are those which show up in those lists most often. For instance, Yamaha PM series mixing consoles have long been a staple on tech riders. Often they weren't the first choice, but they were an acceptable choice.
Or course these things can often end up being somewhat circular. Mixing console A is released from Reputable Company Y and it sells fairly well. Traveling band engineers realize that many sound companies own one so they include it on their lists. Sound companies notice the same console showing up on riders fairly often. Those that don't already own one, buy one so they don't have to keep cross renting from other companies. Lather, Rinse, Repeat.
The whole idea can be somewhat frustrating because Rider Friendly doesn't mean best, or fanciest, or newest, or coolest, or cheapest. But it does usually mean 'equipment that will make us money;....
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