Well with the magazine seated in a Lee-Enfield pattern rifle you can load it quickly with clips. All you have to do is open the bolt and slide a clip in, load it. And repeat for the second clip in order to fully load the Enfields 10 round magazine.

Clips are also extremely cheap; heck I just bought five online for $5.00 with shipping. While a magazine could cost in multiples of excess of that.

In addition clips are very light weight compared to multiple magazines. Which are also comparatively bulky. As a former soldier I'm sure you would appreciate anything that saves you haven't to lug several extra pounds.

Further with a bolt action rifle (even the famous Enfield) accuracy and range are prized more over rapid fire. That is why militaries adopted machine guns. So lacking multiple magazines for quicker firing is not as large of a concern to a pre-1950s army.

TL;DR

Clips are cheaper than magazines.

Magazines were only removed for drill and cleaning. Some early Enfields had their magazines chained to the rifle to prevent a squaddie from removing it entirely.

The reasons is twofold: they didn't want magazines, which cost money, getting lost. They also didn't want the soldier wasting ammunition because they thought it was disposable due to it's easy to replace nature. It was all part of British Army doctrine of the day, which was partly why they included magazine cutoff lever.

The idea was: The soldier would fire single shots, reloaded from a pouch on their waist or etc, and then when the enemy was close the magazine would be engaged and the soldier would fire using the entire magazine.

Doctrine. Check into the idea of a magazine cutoff. It's antiquated thinking but they were completely content with stripper clip loading. I'm sure there was some economics involved, but I think most of it was just they hadn't adapted to the technology yet, magazine cutoffs is the best supporting example of that.