I think it's because Poirot doesn't have nearly as many fans and admirers as Sherlock does.
Sherlock was a brilliant detective who could deduce pretty much your entire life story from looking at your shoes. He had his personal demons, and even took an illegal substance. He's the picture of what kids want to be like when they grow up to be detectives.
Poirot was....different. He had style and class, there's no doubt about that, but he didn't get his hands dirty sometimes like Sherlock did. He had other people do the hands on work. Poirot only had a meticulous nature, everything had to be just so. Sherlock reveled in chaos.
When they were both put on the small screen, Poirot's actor portrayed him beautifully, he nailed the character and he was very fun to watch, but he only had the one actor portray him on television, David Suchet. On the big screen and stage, there was a lot of them, including Peter Ustinov.
Sherlock Holmes....oh my god there was a bunch. However, the definitive Sherlock, the one that did almost all of the original stories but was cut off by his untimely death, was Jeremy Brett. I wholeheartedly recommend watching a few of those, they're amazing.
Sherlock has had some more recent returns to modern times, thanks to the tv shows, Sherlock, Elementary and the Sherlock Holmes films
If Poirot comes back, not just the David Suchet versions, but with a new look and actor, than Poirotians will be forged out of the younger generations.