Price drops only come if a system doesn't sell well or its far into the system's life. The 3DS only dropped price because it wasn't selling well. I mean how long did it take for a Wii price drop? I actually don't know but I didn't think it was months or even a year after the system launched.
price drops happen, whenever it makes the most sense.
the wii, at least in germany, was already dropped in price in the first weeks, when different shops competed for the buyers. and it had its prices lowered everywhere pretty evenly before the first year was over. (only by like 20 or 30€, or it had a game added (basically 30€ off), or another free controller (almost 50€ off), things like that)
as for the 3ds, it didnt exactly sell so bad that they needed to drop the prices.
the 3ds price drop was a reaction to sonys low price for the vita.
obviously, at that moment, they could have kept their normal price and ignore the people saying "what? for just a few bucks more, in about one more year, i could have this more powerful gaming device?! fuck it, i'll wait" but they didnt. they lowered the price so that those people would be able to say "k, now the 3ds is in a good price range, in relation to what the vita will cost. now i dont feel ripped of and can buy it maybe." also they opened up the marked for people with less money. it was a pretty good move, even if it was forced by sony.
in addition to that, the vita basically had its price lowered before it was even released. and i can only guess they went that far (meaning loosing money with every sold console) to put some pressure on nintendo.