Why to use lower case "1080p" and "1080i" but not capital "1080P" and "1080I"? I'm curious about why we mostly use and see "1080p" and "1080i" over "1080P" and "1080I"? (and the same on 720/480/360) Does anybody know any professional reason of that?

I've seen some capital ones, but mostly with lower case, I'm wondering the reason why, but didn't find an answer.

For example, 


Wikipedia uses 1080p and 1080i，there is no single 1080P or 1080I in these two pages
European Broadcasting Union use 720p, 1080p, 1080i in their tech doc
Netflix uses 1080p, 720p in the Help Center(Resolution under Netflix Features)
Youtube uses 1080p in their Help Center
Apple Supports also uses 1080p in their doc, HDV use 1080i / 720i on the logo
Google and DuckDuckGo prompts keywords with only 1080p even I input 1080P!


For above examples, I refer to some huge companies or organizations, but to be honestly I didn't browse all their public materials, so I'm not so sure they do only use "1080p" but not "1080P", just want to show the resolve I found.

Really appreciate if somebody can resolve my question, many thanks.
Here are my starting premises.

In the scientific community, There are some standards in the assignation of letters for units.
Normally a capital letter is assigned if the unit cames after the last name of a person, like Kelvin or Newton, and a lowercase letter in the case of just words like the case of a meter.

In the case of the liter, where it is not based on the last name of a person, the usage of capital L is permitted to avoid confusion with the number 1.

There are also some conventions on the usage of acronyms, where you should use Upper Case letters, for example, PPI.

And there are some more on the usage of abbreviates, where lower case letters should be used.



My assumption is that convention 1 and 2 are the most important in this case.   Although in this case, they are not units, the reasoning could apply.
p and i, in my opinion, are not acronyms, so the capital letter would not override the first 2 conventions. They can also derive just from an abbreviation so premise 4 would also apply.

Regarding the K in a 4K resolution, according to one note in the wikipedia it is just an informal way to say thousand, but with an implied unit, in this case the implied unit could be a pixel, 4kpx. It is based on the idea of the kilo but does not use the standard notation.
But something curious is that it is the common practice to denote a Kilobyte and not kilobyte. Probably to make it more uniform with some other prefixes in Uppercase, like Megabyte and Terabyte.
One idea about prefixes was that lowercase prefixes were used to define smaller than the base unit, and Uppercase to define greater than the unit... but the kilogram has a really strange history. Add that the unit for temperature is a big K.
It could also be just a gimmick. We were used to see the capital M (based on the standard prefixes) regarding resolution, for example, &quot;a 24Mpx camera&quot;. Using a lower case letter probably has less impact than using a capital letter.
&quot;Goodbye to the old 1080p, Wellcome the 4k&quot;
&quot;Goodbye to the old 1080p, Wellcome the 4K&quot;