Why isn't Olaf the one that made an act of true love for Anna?

According to Mark Henn, lead animator on Frozen, Olaf's love isn't true love, but rather a "naïve, childlike love and affection".

Henn: Oh, well, there are a lot of things. It's not always very obvious in the stories, but I think whether it's obvious or not, I think one of the aspects is the whole notion of the different types of love that are demonstrated, are portrayed in the film. You have a variety of types of love shown from Olaf, very naïve, childlike love and affection, to Anna's very reactionary, very seemingly true love, but it's a bit shallow when she meets Hans.

As such, his love isn't the right kind of love to break such a serious curse.

This is backed up in a conversation with the producers

If Elsa could still create Olaf, maybe the loving sister she once knew is still there. He represents a child-like innocent love and animators gave him toddler-like qualities (outstretched arms and the way he moves) to enhance the theme.

A conversation with the Directors

“In ‘Let It Go,’ the first thing she does is the last thing they did, in terms of the last time she was happy,” Lee said. “Like, they built this snowman, not magical, but together—and that was her happiest moment with Anna. And then everything went bad. So when she starts ‘Let It Go,’ she goes right back to the last moment she was happy. And it was Olaf. So to us, he’s imbued with the magic of innocent love, of love that’s pure, that’s undamaged and unhurt by life.”

An interview with the film's Producer

That song gave us the theme of the movie, which was 'love versus fear,' where Elsa is ruled by fear, and Anna is positive and believes in everyone, believes in herself and doesn't let anything get in her way... We ended up rewriting that entire movie so that every character would reflect that theme. So Olaf represents innocent love, Hans represents that love at first sight, there's true love's kiss, and Kristoff represents that love that develops over time once you get to know someone.

And with the best will in the world, Olaf isn't a family member, at least according to the Directors

Jenn: It all leads to the greatest of all [love] which is family. We liked having the parallel stories that combine at the end, because I think that's sort of what it all ends up being.

I don't know of any cannon answer either. My interpretation was the "act of true love" needed to be performed by Anna or possibly Elsa. We know an action by Anna can break the spell because that is demonstrated in the movie. We also know that Elsa can use love to melt all the ice from the storm she created, so she probably can melt the ice she placed (inadvertently) in Anna's heart.

It is clear from the movie that Olaf's love¹ wasn't the key to breaking the spell. Whether this is because only Anna and possibly Elsa could break the spell in unclear. Furthermore, it seems like Olaf is incapable of expressing anything but love and joy. If he is only able to express these emotions then it is possible that his actions aren't considered true because he doesn't have the ability to do anything else.

Out of universe

It seems clear to me the whole "kiss the prince/male lead" was a poking fun at and unraveling the common trope where the damsel in distress is rescued by kissing the male lead. Several Disney princess movies follow this trope (Snow White, Sleeping Beauty, and The Little Mermaid are prime examples). In many ways Frozen is a movie that features and promotes female empowerment. Due to the female empowerment themes in the movie I think it is clear that the central conflict must be resolved by a female.

As an aside, if my daughter is going to be enamored with a Disney princess movie I'm glad it is Frozen due to the female empowerment subtext.

1: Originally I classified Olaf's love as "true platonic love." Richard's answer makes a great point that Olaf's love isn't deep platonic love. I could never imagine Olaf telling a hard truth to Anna like Kristoff did when he was trying to convince her that she wasn't in love with Hans. Olaf does love Anna to the full limit that he can, but he is too childlike to have a deep love.

I don't know that there is any canon answer. A few possible explanations could include:

• Olaf seemed to be vaguely aware his safety was somehow threatened by his staying to help Anna. However I wouldn't say he knew that if he stayed he'd melt and die. His actions therefore might not qualify as an act of true love, simply because he may not have been fully aware of what was going on or what he was doing. It was definitely an act of kindness and selflessness though.
• Anna on the other hand is fully aware that stepping between Elsa and Hans will almost certainly end her life, thus clearly qualifying as an act of true love.
• Olaf was created by Elsa's magic in the first place. Perhaps something created by her magic can't break a curse from the same source. We simply don't know a lot about what Olaf really is and what level of consciousness and free will he does or does not possess.