This is probably a bit before your time, but we chose the 50p pricing because we asked the community what they wanted, and they told us. And you’re exactly right: customers told us they wanted 50p because it’s simple.
With 50p, if I know I get paid in 3 days’ time, and I need to dip into my overdraft, I instantly know exactly how much it’ll cost me. That might seem silly for folks who are comfortable calculating APRs on the fly, but for many people that simplicity is a big feature and a draw to Monzo.
Tom did a really good write-up of how we made the choice way back in 2017, so I’ll link out to that rather than doing a bad job of trying to summarise it all 
There’s no direct incentive for people to move to Monzo (and often lots of incentive to move somewhere else, like big banks offering £150 to switch). The only way Monzo wins is by offering a product people want. The way we do that is ask them, and then we build it.