Because that isn't his job anymore. His job is to ensure only lawful votes are cast. Alleging something like fraud requires that he meet the high legal standard of proving fraud (keeping in mind that fraud specifically refers to intentional manipulation of votes or voting data, so a specific person or persons or at least entity would need to be formally charged with election fraud). That is completely unnecessary and cursory to his goal of having the illegal votes discounted.

Specifically the campaign is narrowing its best legal arguments in each state to avoid the discussion of fraud all together because there is enough otherwise illegal votes that do not constitute fraud according to the law (for example, unsolicited mail-in ballots, or ballots accepted after election day due to state election officials decisions, etc.) to achieve the results they want.

tl;dr proving fraud is hard and requires the alleged fraud be perpetrated by a specific person or entity, there are better legal arguments to achieve Trump victory than alleging fraud.