recruitment for any clinical trial can take a long time. They are trying to use a patient population that is as similar as possible. So patients would need not only the same genotype, but there is generally a specified FEV1 range. Other medications that may either interact with the test medication or affect the results have to be ruled out. a patients history needs to be somewhat stable to prove that the new med is responsible for any improvement or decline. Different trials have different requirements. I was in an inhaled antibiotic trial where the requirements were so defined it took the team months to find participants, even though many people came to be screened for it.
In addition to all that, the vertex trials are very time intensive. Since it is an 18 and older trial, many adults may have trouble fitting it into thier schedule.
All that being said, I think all clinical trials are incredibly valuable and one of the reasons I am as old as I am (37) and doing as well as I am. I would rework my schedule to participate in as much research as I can.
one other reason recruitment may take a while, is that both the centers involved and the drug companies want to be sure there are no mistakes or issues. When a drug is first introduced to the CF population, it needs to be done slowly, until the relative saftey of the drug is more well known. You wouldn't want to take an unproven drug and give it to a large population all at once, if there were bad consequences they would have just been multiplied.
hope that helps, and THIS IS ALL SPECULATION I don't have any inside information that I am basing any thing on, just my own experience fom life and my job.
It's just common sense based on how the trial process works. I took more time to read carefully through the allowing/excluding criteria for some of the trials I am particularly interested in on the clinicaltrials.gov site and you are right, it takes a lot of factors to line up just right.