Because it can be very distracting, and I beleive works against the regulations regarding advertising times.
I suspect this is going to make Commercial TV in the UK just that little bit worse than it already is, suddenly everyone Corrie is likely to be using Nokia™ mobiles (complete with distinctive tone), the IT Crowd will have Cisco™ networks featured prominantly on every bit of networking gear, Ross Kemp will be running around in his Nike™ shoes etc.
I was watching Eureka the other day and the number of lingering shots on Cisco gear (complete with the logo) was silly, same with 24.
Tis was used in Transformer 2 too, speaking of which Michael Bay is notorious for it, too the point where he dedicates whole shots to the product. Horribly distracting.