Aero, it’s an interesting question and the answer is not always that obvious.
Couple of possibilities just off the top of my head in random order.
1. Operator SOP, company prefers you land with flapsetting x for whatever reason. Their name is on the tail and their name is on your paycheck so you do as you are asked.
2. The 737 has gradually been stretched and stretched over the years. Now this doesn’t only apply to the 73 but to more aircraft: certain weights with certain flap settings increase or decrease the clearances for the tail during take off or landing.
Hence this weight or above this flapsetting.
3. Convenient/ no need to use full flap on a 12,000’ runway if you need to exit at “D” for minimum taxi time to stand 34.
4. At my company the bean counters have decided it’s cheaper and faster to replace brakes then it is flap components so the company recommendation is flaps 25.
I still use flaps 30 when we are very light as it’s simply fun to land a humongous plane at 120kts.
As an illustration to the possible forces and wear and tear: