Personal pronouns in English are traditionally gendered ('she' or 'he'), but the quest to find non-gendered equivalents ('e', 'nem', 'per'...) has a long history. Using 'they/them/their' to refer to a person of unknown gender has been common usage for centuries, and in the last few years, this has helped it rise to mainstream awareness as the most popular personal gender-neutral pronoun. 'They' is the pronoun I use (or rather, ask others to use for me), although to be honest I prefer 've/ver/vis/vis/verself'.
A cisgendered friend once tried to convince me that English already has a non-gendered pronoun that would do just as well for people as objects: 'it'. Samuel Taylor Coleridge may have agreed with him, but referring to a person as 'it' is generally considered a serious insult—one that is frequently directed against trans, nonbinary and gender non-conforming people, and often signals violence. You should never use it unless specifically asked.
I do know a couple of people who have said they prefer 'it' pronouns be used for them. Both of them also accept other pronouns, and understand that 'it' can make people uncomfortable. In trying to get into the habit of using 'it' for these specific people, I've found it more difficult than for other pronoun sets, and had some thoughts about why.
Firstly, of course, there's the taboo against using 'it' for people, which is stronger yet with an awareness of its role in cissexist violence.
Secondly, I feel that 'it' disappears in speech or writing even more than pronouns usually do. I think this is because it can so often refer to abstract or even undefined entities, while, in English, 'he' and 'she' almost never do. What is the 'it' in the statements 'It's raining', 'It's true that...' or 'It's been three weeks since...'? Although pronouns are often unstressed, 'it' feels hard to stress in places where that would be necessary. This is quite similar to objections people sometimes raise against the singular they, and I think it would get easier with practice.
Most surprisingly, perhaps as a direct result of the taboo, using 'it' for a real, specific person feels strangely intimate to me, like the transgressive play that can take place in a strong and safe relationship but not outside in general society. Referring to someone I know, but less intimately, as 'it' feels like claiming a greater role in their life than I have.
I of course intend to continue using the correct pronouns for people and switching as the correct pronouns change, even if it's difficult for me. It can be useful (and interesting) to tease out the reasons why adopting different pronoun sets are more difficult than others, so that's what I tried to do here.