Bottom brackets have a fundamental flaw that the most typical shell size is too small for a good quality axle and good quality bearings at the same time. All bottom brackets fully inside the shell thus fail very often, usually due to bearings or axle breaking.
Typically, the higher the load is, the more likely is the failure of axle or bearings. So, gearing affects but your weight affects too.
Shimano has outboard bottom brackets in Hollowtech II components. These have the bearings outside of the bottom bracket shell. Thus, with Hollowtech II, you can have both good axle and good bearings.
I think that with Hollowtech II, your bottom bracket worries should be mostly gone. What is left is crank failure. The crank-pedal attachment has a fundamental flaw too, it cannot keep the pedal non-moving. Microscopic motions slowly damage the pedal eye of the crank, to result in a fracture of the crank. The pedal eye design should be modified to have a countersink and the pedal spindle should be modified to fit to the countersink. With traditional pedals, the crank can be modified by adding the countersink in a machine shop and a split collet can be made to fit to the pedal spindles: http://pardo.net/bike/pic/fail-001/FAIL-016.html
The countersink and manufacture of the split collet is probably more work that you are willing to do, so eventually your crank fails -- especially in a single speed bike, if you are a heavyweight rider. If the bottom bracket is of the Hollowtech II style, it will probably outlast your cranks.
You might be interested in this online museum of broken bicycle parts: http://pardo.net/bike/pic/fail-001/000.html
Perhaps the bottom brackets I have had were of low quality?
Were they of the square taper type? If so, they are low quality by design. Toss those away and select Hollowtech II.
In general what factors are involved in the usual degradation of the bottom bracket?
- Engineering (type of your bottom bracket)
- Price (affecting the quality of bottom bracket materials available for the price point)
- Your weight
- Your riding style, spinning at high speed versus mashing at low speed
- Tire size and air pressure (affecting suspension characteristics)
- The non-uniformities of the road
- Whether you approach non-uniformities of the road sitting on the saddle or standing on the pedals
- Whether or not you ride in a hilly / mountainous area
- Whether or not you use drop bars from which you can pull up using considerable force allowing you to pedal with more force
- Whether or not you use clipless pedals allowing you to push up using the rear foot to result in more force at the front foot.