IIRC there were two reasons for this. One was an information-leak concern whose details I don't fully remember. The other is flow control. A server push doesn'

IIRC there were two reasons for this. One was an information-leak concern whose details I don't fully remember. The other is flow control. A server push doesn't automatically send the entire object. It opens a new stream and fills the window, but the client must actively read / ack the stream for the remainder to be sent. To do otherwise would allow the server to de-facto violate the client's stated flow control settings and potentially DoS their network link.