Why did Daito have to die?

The answer to your question is that Daito was a threat to IOI. In fact, all five of the gunters at the top of the Scoreboard were considered a threat to IOI. The organization's main goal was to annihilate these threats to their dominance.
In chapter 30, the text tells us about Wade hacking deep into the Sixers' private database. He comes across five folders enclosed in what is called a Threat Assessments folder. The five folders include personal information about Parzival, Aech, Shoto, Daito, and Art3mis.
Wade discovers that IOI has been spying on Art3mis and Shoto. Because IOI owns hundreds of local telecom companies, it is effectively the largest Internet service provider in Wade's world. From this formidable position, IOI has been able to keep Art3mis' home under surveillance. Wade learns that Sorrento was planning to abduct both Shoto and Art3mis (in real life). Once the two were in IOI's power, Sorrento was going to force them to help IOI open the Third Gate. After the Third Gate was breached, IOI would simply "dispose" of both gunters.
The only reason Parzival/Wade has not been caught is because he has taken the trouble to lease a private, direct fiber-optic connection to OASIS (the Internet in Wade's world).
So, we can see that Daito had to die because he was considered a threat to IOI. The organization had targeted all five gunters for death.