There's some pretty straightforward economics* in play here, too, Colin.
Let's say you want to commission a story consisting of 120 pages of original full colour strip. You commission it as a single work OGN. The writer writes it. Chances are, he'll** have written the whole thing before the artist starts work on it and will want paying. Let's say he's getting £50 a page, so you have accounts raise a cheque for £6000.
The artist starts work. Let's say he can do page of finished full colour every two days, so it's going to take him 240 working days. He'll probably intend to work 7 days a week, but in all likelihood real life will mean he'll only do 5 days a week (in fact, why shouldn't he do a 5-day week like everyone else?) meaning that the job will take him 48 weeks, assuming no major illnesses and the fact that he's happy to go a year without taking a holiday.
You've probably arranged to pay him in instalments on completion of sections of the book (or he'll starve). Let's say you've agreed £200 a page and that he'll complete 40 pages every 16 weeks. You greenlit the project in January, the writer delivered the script at the end of Feb, and the artist started work in early March. On 1st July, he delivers the first batch of pages. You get accounts to raise a cheque for £8000.
Accounts notes that this project has been ongoing for seven months, cost £14,000 and delivered not one penny in revenue.
On 1st November, you get the second batch of pages. The project is now in its eleventh month, has cost £22,000 and still delivered no revenue. Sometime in January, you get the last batch of pages and issue the last cheque. The project is now £30,000 in the red and has brought in nothing. Now, you have to add in the costs of marketing it, and try to recoup all the money from that single product.
The alternative is to commission and publish it in 5-page episodes as part of your established anthology comic. 24 episodes, spread over two 12-part series. If you have control of your workflow, you can probably work as little as eight weeks in advance, meaning that you are raising cheques to the writer for £250 a week and to the artist for £1000 every fortnight and are making money back on the material before they've even finished Book 1.
In the unfortunate event that the artist becomes ill, the OGN essentially goes on hold where the weekly schedule will probably accommodate a fill-in artist for an episode. If the series proves disastrously unpopular, you can can it after the first series and (the beauty of an anthology) you probably won't even have lost any money.
If it's wildly popular, you can stick it out as a collected edition anyway, certain in the knowledge that 1) it's got an audience, and 2) you've already paid for the material so everything over and above the printing and production is basically free money.
*Any numbers quoted are entirely hypothetical and have been selected for the purposes of making the maths easy.
**Or she. I'm saying 'he' because it's quicker to type.