Morning Miller,
Basically, Five have been slowly introducing their US dramas, so that they:
a) don't overload viewers with too many new shows at once
b) don't waste all their new programming in one go
c) have seen how the ratings were for "Law And Order", and "Law And Order: Special Victims Unit" before they went ahead and bought "Law And Order: Criminal Intent".
Sometimes US shows get shown here almost immediately after they air in the US, because the rights to the shows were snapped-up on the basis of word-of-mouth from US critics and favourable ratings in the US for the first few episodes. However, sometimes, some shows that work in the USA do not work over here, or vice-versa. Hence, some TV stations will hold-off and wait and see how the entire season has done, before buying the rights to it.
With "CSI: Crime Scene Investigations", Five bought the rights, after having favourable US reviews and word-of-mouth. Not to mention the pedigree of the cast and crew, and the fact it was "different" to previous crime and police dramas. PLus, there's the added fact it was an intelligent drama that required people to think and pay attention to what was going on, rather than like some shows where a child could follow the plot.
After it became so successful on Five, they went right ahead, and bought the rights to Season 2. Then, again, after it became Five's most popular show, they have continued to buy the rights, for each subsequent season.
I hope that answers your questions.
Pooch