The obvious reason is that people; as has been endlessly pointed out, don't tend to vote on the basis of the Saturday night dancing; in general not necessarily aimed at any FP's, but on who they fancy, feel sorry for, have a grudge against, think is funniest when dealing with the judges, tabloid rumours, etc.
Sadly, it's one of those things that goes hand in hand with any reality TV series. When the BBC came up with the idea for SCD they wanted to get away from the old Come Dancing format, of only the judges being able to decide the winners, & get the public involved with what was happening on their TV screen every Saturday night, so the public vote gets equal weight with the judges scores, at 50% of the total mark (but is used as the deciding factor in the event of a draw). As they also wanted to raise as much money as possible for CIN (nothing wrong with this desire, BTW), they decided to open the phone lines again, immediately after the results are announced each week. Ineveitibly, this means that we get the BB syndrome; where you seem to be able to vote on the basis of who is the least boring to watch sleeping (after all those tedious hours of live coverage, in which the contestants don't actually do anything).
Those of us who base our votes upon the performances just don't stand a chance of influencing the result by waiting to vote until after the main show on a Saturday.