You're right that the Ramones played pop songs. They were heavily influenced by the Beatles (they took the Ramones monikers from a pseudonym once used by Paul McCartney), surf music and girl groups. However, the band played their songs at a breakneck tempo that was unlike anything else at the time. The Ramones showed up to their early shows at Max's Kansas City and CBGB with leather jackets and torn jeans and those mop top haircuts, playing these blistering songs; the longest song on their debut album was 2:35 and their average show in 1974 was about 17 minutes long. Practically overnight, bands were getting leather jackets and pushing the tempo of their songs. The bassist for the English band Generation X said

When their [debut] album came out...all the English groups tripled speed overnight. Two-minute-long songs, very fast."

The speed and simplicity of the Ramones was a direct reaction to the overwrought stadium rock that dominated the early and mid-70's as well as a perceived lack of rebelliousness and toughness in rock.

Tommy Ramone:

In its initial form, a lot of stuff was innovative and exciting. Unfortunately, what happens is that people who could not hold a candle to the likes of Hendrix started noodling away. Soon you had endless solos that went nowhere. By 1973, I knew that what was needed was some pure, stripped down, no bullshit rock 'n' roll.

The founder of Punk magazine:

punk rock had to come along because the rock scene had become so tame that [acts] like Billy Joel and Simon and Garfunkel were being called rock and roll, when to me and other fans, rock and roll meant this wild and rebellious music

The early punk scene was more diverse than is usually remembered. Arty punk bands such as Television and Talking Heads and more pop oriented acts like Blondie also were regulars at CBGB and Max's. They all made major impacts as well. In some ways, they made perhaps an even more immediate impact in the US than the Ramones as punk quickly transitioned to post-punk by the late 70's. However, by the early 80's, kids in Washington DC and the suburbs of Los Angeles who had missed the original wave of punk rock reacted against the excesses of new wave and post punk (and their anger at Reagan's America) in the same way the Ramones had by stripping down to the bare essentials and cranking up the speed and attitude. Bands like Minor Threat, Bad Brains, the Adolescents and Black Flag ushered in the era of hardcore punk and helped to firmly cement the punk rock formula as one of loud, fast and simple.

By the late 80's and early 90's, this style had settled into a somewhat less extreme and more mainstream sound spearheaded largely by California bands like Bad Religion, Pennywise, Green Day and the Offspring. Punk famously went mainstream in the mid-90's when Green Day, the Offspring and Blink-182 exploded in popularity and even relative unknowns such as Jawbreaker were snatched up by major labels. Bands in the riot grrl scene and many other pop-punk bands that fell on the more punk side of that term such as Screeching Weasel and the Queers were unabashedly imitating the Ramones. By this time, the Ramones had already long been ensconced as godfathers of punk that had defined the sound and look. Leather jackets are still ubiquitous in the scene and three chords, fast is still the template a million punk bands aspire to.

Ultimately, what made the Ramones so influential was the same thing that made Nirvana so great; they wrapped timeless, perfect pop songs in an entirely new packaging.