In withholding their own data (or any other attack) block propagation would be slowed and so they lose any race.
For someone to lose a race, there has to be have to be competing chaintips in the first place, and the effect of propagation delay is often overestimated.
I don’t think there was any real incentive to use [OP_RETURN] but people moved because its designated.
OP_RETURN was cheaper than embedding data into payment outputs. E.g., a P2PKH output takes 34 bytes of which the 20-byte hash could be used for data embedding. For a payload of n an OP_RETURN output takes 11+n bytes with the original policy limiting n to 80 bytes. OP_RETURN has less overhead for any payload length.