Neon Genesis Evangelion was a landmark anime for various reasons, with the main ones being the tone and themes of the series. Full of deconstruction and the grim realities of what a Super Robot series would look like in a realistic setting, the series was rife with symbolism and is a deep dive into its cast's continually degrading mental health.
Despite the heavy nature of the series, it continues to be a popular franchise even with people who haven't seen the show. It's one of the most marketable and toyetic anime series in existence, with its characters and concepts having a host of toys, video games, clothes and collectibles. However, it may be because of how dark the anime is that fans continue to buy Neon Genesis Evangelion paraphernalia.
The Mecha Effect
Evangelion is still at its core a mecha series for all of its deconstruction, psychology and existentialism. It bridges the gap between Real Robot and Super Robot anime, but this only makes the action between the EVA units and the Angels that much more exciting.
Mecha toys from franchises like Gundam are some of the most popular and money-making collectibles in Japan and with international anime fans. Evangelion is no exception to this rule either. The now-iconic EVA units are incredibly marketable given their distinct differences from the looks of the RX-78 and the VF-1 mecha of its contemporaries.
Evangelion is known for its strong female characters, namely Rei and Asuka, who were far more trustworthy and reliable than the indecisive Shinji. Asuka helped popularize the "tsundere" trope, while Rei is essentially the same thing to the not quite as ubiquitous "kuudere" trope. This makes the two young heroines extremely important to the development of modern anime writing.
Combine this with their striking designs and suits, and it's obvious why merchandise of them would sell. Rei and Asuka cosplayers are much more common in the convention scene than Shinji lookalikes, and it's owed to how much more popular their characters are. Gainax can make a quick buck off of otakus locally and internationally by slapping their likenesses on toys, posters and even body pillows.
Defanging the Angels
As mentioned, Evangelion is often a dark, depressing and subversive series. Even amidst the epic action between the gigantic EVA units and the Angels, death, destruction and very visceral human fear loom in the background. This tense tone, when combined with the Biblical mythology behind the Angels, their inhuman forms and Shinji's constant screams, turns the series into as much of a psychological horror series as it is a mecha show.
This is a far cry from the Gundam franchise, which is much more straightforward with its action and tends to have political backdrops to their plots. For this reason, it makes sense that fans would want to enjoy Evangelion in a way that does away with all of that heady stuff. Some of the more serious elements of the series are a lot easier to parse when there's a sillier or more irreverent incarnation of it to consume.
The popularity of Evangelion entices fans who may otherwise not be looking for a grim series. While they may not always want to dive into the series dark themes, people still seek out tangible ways to interact with the franchise. Thus, the Evangelion marketing machine that churns out models, games, chibi figurines and even food products.
With more universal entry points, fans can be introduced to Evangelion through products or partnerships -- like the EVA-skinned shinkansen train or surprising kids' TV show crossovers. The franchise's multiplicity is the best reflection of the realism and humanity that it attempted to portray. While the world is full of serious and sometimes grievous issues, it also has numerous trivialities that keep things fun amidst the darkest angels plaguing humanity.