The cost of the s1s did not drop really low until after the s2s were sold out. When the s2s were selling the s1s cost was high enough that the s2s were still a better deal. If they were to post the s2s while the s1s were around the 400 each mark then I doubt they would have sold as quickly.
Well, not really. Not trying to be argumentative at all but Bitmain started lowering the price weekly well before the S2's came onto the scene. Consider these facts:
- Bitmain announced the last batch of S1's on Jan 15th for 2.2 BTC each.
- The price had been steadily lowered each week and was 0.996 the day before the S2 was announced.
- Bitmain announces the first batch of S2's on March 14th.
- Price further steadily lowered to the present 0.893.
The decrease before the S2's announcement was 55%. The decrease after the S2 was only 11%. So the S1's were more cost effective on a per-machine basis when the S2's were announced, which was mentioned in a few threads: $3899 for 1TH/s S2 vs ~$3000 for 1TH/s worth of S1's.