I'm not sure that makes sense... If you allready have 13 episodes, why would you sabotage the series, even if you thing that it will be a failure it's not the logical course of action... There's no benefit for anyone...
Perhaps it was just a matter of bad luck and awfull coordination from the networks part.
Although I loved the series and the movie, I haven't done any searching as to why it was cancelled, so I might not know all the facts. But that could only happen if Wheddon had pissed off a lot of people at the time and this was personal. From a business point of view, it's kind of wierd...
Why would you commission a show and then not give it more than half a dozen episodes? Why would you show the second show before the first, thus not establishing the characters as intended? Why would you move the transmission time around with little notice?
There are lot of reasons for these things. Internal politics, poor planning, bad judgement, lack of promotion. The favoured reason is that they simply changed their mind, and decided they didn't want the type and style of show as they had commissioned it. Rather than some exec admitting they screwed up, they ensured the show would fail so that it wasn't managements fault, but a fault with the show itself. If TV stations were infallible, every single show would be a big hit, but it isn't. There really isn't some magic formula that can be followed and that every exec knows what they are doing.
You only have to look at the TV schedules to see that this type of thing happens all the time. So much TV production is effectively outsourced to external production companies, it's easy for the networks to commission stuff, and then kill it off with little expense or commitment to themselves, so that's what they do. Like I said, it's just scattergun TV making, throwing a much as they can at the audience and seeing what sticks, almost with little understanding.
I was reading an article on shows that consistently get cancelled from the 10 pm Monday night slot. There are low audiences there, and so the networks keep putting shows in there and then cancelling them when they don't do well. The networks don't understand why they can put on a good show at 9pm, and yet that large audience turns off at 10 pm, so they keep cancelling shows that loose 10-20 percent of the 9 pm audience instead of staying with their 10 pm slot. It must be the shows right, so lets keep trying something new.
Turns out there's just a general reluctance for people to stay up till 11 pm to watch a show on a Monday night. They've just had a weekend, they've got a week of work ahead of them, and they want to get an early night in. It's as simple as that , and so instead of getting an audience into that slot, you're actually killing off any show that you put in there. The exec's logic is reversed because it's not a bad show that's losing the audience, it's a bad slot that loses the audience.