Lower limits tend to ensure that all passengers will find room for their carry-on items. Higher limits tend to make passengers with elite status and others who are entitled to priority boarding happier, at the expense of those who board toward the end and must often put their carry-ons some distance from their seats or gate-check them. It's not possible, given the current state of carry-on storage, to do both. Airline management can lean toward one side or the other, keeping in mind that too radical a departure from previous practice will upset many people - including some whom it helps.

Another factor is that enforcement of stated limits also varies widely, from airline to airline and often from station to station or person to person within a given airline.

Smaller allowable carry-on's mean aircraft can be boarded a lot quicker than is the practise in USA with effectively unlimited carry-on.

And it also depends on why the bags are heavy. If you have a good explanation that the airline believes and accepts, you may get waivers. If not, you may have to check the bag, if the weighing is being done while you check-in in person for your flight at the check-in desk where they have plenty of weighing stations. It may therefore be that it's less likely if you check in online (and go straight to the gate where they're less likely to have a bunch of weighing stations), but for some international flights it may impossible to check in online.

And I would guess that some people who want to ccarry-on heavier stuff may choose why airline to fly based on that. For example, you can fly between much of South America and the USA on either LATAM or AA, and with AA you might have higher prices sometimes but no weight limit on carry-on, while LATAM may have better prices sometimes but does have a limit on carry-on.