This is an interesting one.
Lemme start by throwing out some background to this problem:
There are two paradigms of running a payments network: most payment cards these days (Mastercard, Visa, American Express, …) use what’s called a “dual-message” system, while Maestro is the most prominent “single-message” card these days.
Single-message is really simple: when you walk into a store with a Bunq card (Maestro, single-message), you swipe your card, and the money moves from your account to the merchant’s.
When you walk into a store and buy something with your Monzo card (Mastercard, dual-message), on the other hand, the merchant doesn’t actually take the money out of your account straight away - they make an “authorisation”, which is basically calling dibs on it; it’s deducted from your balance, but if you tap on the transaction, it shows as “pending”*. A week or so later, they come back and “present” those transactions, at which point the money actually leaves your account.
The reason this more complicated model exists is that there are some neat things you can do with it, like…
- Updating the authorisation as you spend; pay-at-pump gas stations do this.
- You can make purchases (“offline authorisation”) when you don’t have internet access, such as when you buy snacks on a plane; the airline can remember what you bought and charge you once you land, when they can get proper internet access again.
- You can be TfL, authorise for a minimum fare, tally how much you ride for in a day and then claim it all the following morning. (If you ride the tube with a single-message card, you actually instead get charged the fare cap upfront and then they refund you what you didn’t ride for.)
An interesting thing to note is that while these are separate networks, you can actually run single-message transactions over the dual-message network and vice versa; in the case of Mastercard and Maestro, if a single-message merchant sends a single-message message to a dual-message Mastercard (try saying that ten times fast), Mastercard will translate that into an authorisation immediately followed by a presentment to simulate the single-message behaviour (I’m not super familiar with how this works in reverse). So in theory, your dual-message Mastercard should be usable with either kind of merchant.
Now, a card can have multiple “applications” on it, advertising different capabilities that the card has. For example, it’s not unusual for UK debit cards to be able to act both as a Mastercard/Visa card, and a “LINK” card, for use with certain old ATMs.
And herein lies the catch: while your Mastercard card can be used with either kind of message a lot of merchants in places like the Netherlands 
and Belgium 
have terminals that look at your card and go “what the heck is this Mastercard nonsense, that’s not Maestro, I can’t talk to that”… even though it technically could, if it tried, and indeed we’ve never seen any issues as long as terminals know to do that.
This is a tricky problem to solve. If we wanted to issue Maestro cards, we could definitely do that, but we’d have to spend a lot of time implementing the single-message system properly (instead of relying on the automatic translation), and unfortunately there are only so many hours in a day 
. I’ve theorised that it’d be possible to make a “hybrid” card which presents as both, but actually producing such a card turns out to be technically tricky. Ultimately, the problem will likely fade over time as merchants upgrade their terminals to ones which correctly detect Mastercard as a thing they can talk to, but it’ll be a good while before that happens everywhere. 
* Some banks make a distinction here between “balance” and “available balance”, the latter being the one with authorisations deducted. I never really understood why they make this distinction.