The reason for the public keys to be able to be compressed or uncompressed is due to the property of the ECDSA curve. The curve has the x and y coordinates but the compressed keys only specify the x coordinate and the client can figure out what is the correct y coordinate, of the two possible coordinates. This isn't computationally intensive and its easily implemented.

With transaction, it is a different case. If you were to compress it like that, the client would have to uncompress the transactions, every single time they validate it. Same thing with when someone mines a block. A side effect of this is the increase in the rate of transactions, which wouldn't be good, considering the tradeoff for it.