Is it hard to scissor cut ?
Yes, Bichons are generally difficult to scissor-cut (every angle of a perfectly groomed Bichon is rounded; there are no hard edges/lines).
For someone

who is not certified by the National Dog Groomers Association of America, Inc (NDGAA) as a NATIONAL CERTIFIED MASTER GROOMER (NCMG)

who isn't adept with the scissors (ie. using clippers/shavers all the time)

who does not have a proper knowledge or even an inkling of how a Bichon is suppose to look like

who has never scissored a Bichon before
... the task of scissoring a Bichon IS DIFFICULT.
Why I mentioned NCMG is because a local groomer I know here, returned from the States with that certification. While undergoing training, these aspiring groomers learn to clip/scissor most breeds (including Bichons). So I would imagine someone with no proper training, no certification, would certainly find it hard to scissor a Bichon!
That said, someone who is not certified by NDGAA may very well be adept at scissoring Bichons, because over the years, he/she has learnt to do it (with the various Bichons that go through his/her 'salon' doors).
More important than a certification IMO, is the innate ability of the groomer to style a dog. He/she must have a 'vision' of how the dog is to look (understanding the breed standards is a bonus!), and with that 'vision' in mind, proceed to 'sculpt' the 'specimen' -- in this case, a Bichon to look like the powder-puff that Frank Sabella pioneered for the breed.
Why do some groomers like to make bichons look like poodles. ?
By virtue of their own insecurities/inexperience (at not knowing how to properly scissor a Bichon), these groomers retreat into a 'comfort zone'. The manifestation of which is a Poodle, a Schnauzer or whatever breed it is that they're most adept/comfortable with clipping.
Here locally, Schnauzers are very popular. So much so that Maltese and Shih Tzus come out of grooming salons looking like Schnauzers

. I'm not kidding, seen it countless times. Sometimes, these Maltese & Shih Tzu owners actually ask for the Schnauzer cuts, but more often than not, it's because the only style/cut the groomers know is that of a Schnauzer.
I got the feeling it was a time factor and money factor.
Ten has a very valid point. It takes a relatively longer time to properly scissor a Bichon than to clip it with a clipper. For some groomers (especially those who became a groomer not because he/she loves dogs, but it's just a it-pays-the-bills kinda job), they're just not into spending that kind of time scissoring your Bichon, when the same amount of time can be directed to grooming/clipping 2 other dogs.