I can give an answer from a historical linguistic perspective. I can't give a more direct answer, but I can explain how these changes work in general.

There is a phenomenon called politeness shift, and it's happened many times in various languages. For example, English thou became completely replaced by you, so that now not only does English not distinguish between formal and informal 2nd person singular pronouns, but the distinction between 2nd person singular and 2nd person plural is also lost (unless you're talking about dialects with y'all, but these are discriminated against).

It's not uncommon for a polite 2nd person singular to be derived from a 2nd person plural pronoun with little to no change. This has happened in many European languages (the t-v distinction), but also in Proto-Malayo-Polynesian, where the reconstructed 2S pronoun *kamu (iirc) corresponds to the 2P pronoun in a lot of Formosan languages.

In 15th century Spain, an intricate system began to arise with at least three (and sometimes four) levels of formality. The old polite pronoun vos was pushed down the politeness scale by the newly coined vuestra merced ('your mercy', later shortened to usted). At first it was reserved only for 'people of quality' (high social status), but quickly became the normal way to address any stranger, with vos being reserved for close friends and/or insults. The end result of this is, the multi-layered system has collapsed almost everywhere in the Spanish speaking world, with usted becoming the sole formal/polite 2S pronoun, and the informal/familiar pronoun occupied by either tú or vos, depending on the dialect.

The change of politeness is almost always unidirectional. Another example of this is Japanese, where the 2S pronouns omae, temae, and kisama started off as polite, but are now extremely informal to the point of rudeness or even insult (especially the latter two). Euphemisms in general also suffer from this semantic process, where they become less and less euphemistic and more crude up to the point where a new term has to be coined (seen commonly in taboo topics like personal hygiene, sex, race, etc.).

The most important thing in all of this is that people almost always prefer to err on the side of caution when it comes to politeness and formality, thus inadvertently causing these semantic shifts.

tl;dr: words change their meanings from more polite/formal to less polite/formal.

a minor correction. a baronet or a knight bachelor (people known as Sir X) are not nobility, that is for barons and above. (in the UK at least)

My previous answer

A commonly accepted theory is that 'Sir' is related to 'Senior' and it was used as a marker of polite respect before it became used in the more narrow official sense of a knighted man.

That is to say that the usage OP assumed to be newer is probably the older usage.