Because the secret ballot is a basic democratic principle.
In the polling station the timid voter is protected from interference by the supervising officer, who makes sure that noone but the voter sees the mark on the ballot paper. With a postal vote there is no such protection. The household or community bully can insist on seeing the vote made. It is totally inconsistent for secrecy to be enforced in one case and left to chance in the other. The wide distribution of postal votes to all who request them makes things easy for those who would bully or bribe in order to sway the votes of others. The precious principle of the secret ballot, established in the nineteenth century only after much struggle, is in practice now abandoned. The previous regulations restricting postal votes should be restored. If this leads to lower turnout, so be it. Better a low turnout than a corrupt result.