Its because small party publishers have had business drop significantly (read: pretty much die) since the 4e announcement. They need to do something to keep themselves afloat. If they had a copy of the 4e SRD, they could be spending their time on creating new product to release alongside the core books. It would still be a bit of a downtime, but hopefully something they can live through. Without access to the SRD, they have nothing to do but wait. Many of them cannot afford to wait for the 4e release to start their own product; the downtime will kill the complany. The only other option is to release product for other systems, which means investing significant amounts of time and money into other systems, which means they're less like to spend that time and money (by economic necessity, not by choice) to develope 4e product once they finally get the SRD.