A building’s heat loss (the rate of energy needing to be re-added to maintain thermal equilibrium) increases with decreasing outside temperatures and decreases with better insulation.
A heat pump’s max output in heating mode decreases when the outside temperatures are low enough (whereas a gas boiler has a roughly constant max output and it’s quite inexpensive to size a wall-hung combi for 150K BTU/hr [44 kW] if needed).
Buildings that are fine on the coldest design day with a gas boiler may need more output than a heat pump can provide on that day.
Adding insulation can reduce this gap, which is why you’ll often find a heat pump project needs insulation, while a boiler replacement like-for-like would merely benefit from additional insulation.
Yes, gas has been so cheap, until recently, that nobody has invested into insulation, and our housing stock is so poor quality and small in average size that insulation has lots of detractions in terms of property appearance and internal area.
The UK government back in 2010 actually had major plans for nation wide insulation, but famously the prime minster at the time, David Cameron, ordered the cutting of "the green crap" (widely reported to be his words) to help resolve some short term political problems with the budget in the early 2010s.
Heat pumps have a solid max heat output that cannot scale as easily as gas heating. So without insulation they can quickly hit a limit. The upper limit for gas system is limited by how much gas can flow through a pipe, which is a huge amount of energy. The upper limit for a heat pump aystem is set by the compressor capacity, which is in turn set by the electrical circuit, and relative temperatures.
The piece you're missing, beyond just "gas is cheap", is that increasing the heating capacity of a heat pump costs lots of money when installing it, whereas increasing the capacity of a gas boiler isn't much more expensive during installation, but costs loads more when running it.
So technically it would be possible to install a heat pump with the same capacity as the existing gas boiler, but it would costs a ton more and it would all be upfront.
Another detail I think others might have missed, heat pumps need to defrost sometimes, during which time you effectively can't provide heat. In other words, you need to trust the house to be a good enough thermal battery so that you aren't having to continually heat it
EDIT: ok so the best interpretation I have is that it has just been so cheap with gas that people have not bothered insulating their houses.