Firstly, let's give a bit of context: in 1995, the bands Oasis and Blur had a rivalry which was very well-publicised in the UK - that they both released singles in the same week, and that obviously only one of the two bands could top the chart (it turned out to be Blur's 'Country House' rather than Oasis's 'Roll With It' which won that battle). This was headline news, covered on the news on British TV, etc. The two bands were seen as being the most popular exponents of a pop/rock subgenre, 'britpop', which also included bands such as Supergrass, Pulp, Suede, Ash, and other exponents of what was effectively an updated version of 1960s/1970s pop, and which at the time was seen by many as the UK's answer to grunge.

On the back of this, in the UK, Oasis's subsequent album (What's The Story) Morning Glory, released later in 1995, ended up selling about half a million copies in the UK (15x platinum). Even in the days before the streaming services and/or the iTunes music store, this was a lot of copies: it appears to be the highest-selling original album of the 1990s in the UK (the compilation ABBA Gold beats it though, selling 19x platinum, but that's not an original album); only a very small amount of albums have sold more in the UK.

What's The Story's success was propelled by several prominent singles, of which 'Wonderwall' was the biggest. Singles in the UK tend to be bought in the first week and so they debut high, and then rapidly fall out of the charts. 'Some Might Say' (the first single from the album) spent 3 weeks in the top 10. 'Roll With It' spent 4 weeks in the top 10. 'Don't Look Back In Anger' spent 5 weeks in the top 10. 'Wonderwall' spent 11 weeks in the top 10.

So basically, in terms of why the song became embedded in popular culture, one big reason was that, in the UK, it was the biggest song from the biggest album of the 1990s. So, in terms of wondering why 'Wonderwall' has inspired memes is a bit like wondering why 'Old Town Road' or 'Despacito' has inspired memes - once songs have become that popular, they very often have a life of their own.

Elsewhere in the world, Oasis and 'Wonderwall' were less popular, outside of the somewhat-nationalistic, Anglocentric Britpop context of the UK...but still very popular nonetheless. What's The Story sold 4 times platinum in the US (about 4 million copies) - respectable but not quite the sales of Alanis Morissette's Jagged Little Pill (16 million copies) in that country. In Australia, What's The Story was the 4th highest selling album of the year. Nonetheless, in 1995, 'Wonderwall' was a song from a very successful album, and a song that, therefore, a lot of people who were at least vaguely interested in pop culture would have heard across the world.

As to the 'anyway here's Wonderwall' meme, it's a relatively easy song to play and sing at a fairly basic level of competency. The actual chords in the song are more complex than the typically 'guy playing Wonderwall in the corner of the party' version, and Liam Gallagher's vocal is obviously a convincing emotional performance which the average guy in the corner might not quite approach...but the song has a relatively small vocal range that's well within the range of most male singers, and as a mid-tempo ballad, it's also relatively easy to play and emote to. It also, lyrically, is obscure (the 'Wonderwall' in the title being a reference to an obscure George Harrison instrumental soundtrack record, Wonderwall Music from the 1960s); this perhaps allows people to give the song their own meaning related to the yearning emotion of the vocal melody.

Memes are generally relatively hard to track the origin of, and perhaps the specifics of the Wonderwall memes are a bit beyond the scope of this subreddit, with its 20-year-rule. Some of why things become memes are up to the vicissitudes of randomness; maybe if you rolled the dice again it would have been another song - but in this universe it just happened to be that one. 'Song 2' by Blur also became something of a meme with its simple 'woohoo' refrain being sung by crowds at sports events. But its enormous success in a fairly influential country in the music world, and the characteristics of the song, made it the kind of song that had a larger chance of becoming that meme than probably most other songs.