Because he is obviously expecting that when halving happens, Bitcoin will boom. Many alts will ride the wave for sure, but some will wilt as well and it's better to have 1 sure thing in your hand than 2 that of which only 1 is a sure thing. Holding doens't always work for alts as most of us can attest.
If you can see the advantage on your side then why not but as said i would choose up on having some alts in my stash yet you arent only focusing on single, just be sure not to go all in and split
on several top alts.