This is very common. We have it in Norway too. It started in 1882. At that time, it was mainly farmers who had mortgages, so the idea was to encourage productivity, making it less costly for them to buy new land for cultivation (You might wonder, wouldn't this just drive up land prices? I wonder, too.)
But once you have them, they're hard to get rid of. When they were in charge for 40+ years, Labour defended them because they wanted to push for homeownership so working people didn't have to live at the mercy of landlords. In the 80s I believe they considered changing it, but then the Conservatives (who of course benefit disproportionately from the deduction) were in a position to block it. These days there is an interest deduction on ALL loans, not just mortgages. Why and how that happened I have no idea.
But once you have them, they're hard to get rid of. When they were in charge for 40+ years, Labour defended them because they wanted to push for homeownership so working people didn't have to live at the mercy of landlords. In the 80s I believe they considered changing it, but then the Conservatives (who of course benefit disproportionately from the deduction) were in a position to block it. These days there is an interest deduction on ALL loans, not just mortgages. Why and how that happened I have no idea.