There are real difficulties for any airline offering sandwiches in flight.
1. Some days you sell out and need twice as many, other days nobody wants to know. Unused sammies have to be thrown away at the end of the day, thereby incurring a cost. On days where you sell out, passengers that go without are unhappy, so the airline loses goodwill and revenue. On days where you gash sandwiches, the accountants yell at you. Gashing a relatively small percentage of sammies at the end of the day can easily wipe out your profit margin on the sales that you have actually made.
2. Contrary to popular belief, cold food can often be more expensive to produce than hot food - you can't freeze a sandwich or a salad! Sammies have to be produced fresh and made daily, and then stored at a safe temperature for the rest of the day. They can't be allowed to go stale or soggy. Therefore there are extra costs in the production, loading, packaging and storage of sammies, which bumps up the cost, reduces the profit and reduces the volume of wastage you can accept before the profit disappears.
3. Choice - Not only is there an issue about the number of sammies carried per sector, but also an issue about the selection. You can also disappoint a passenger by not having available their choice of filling or bread.
Now, in the light of the above, you begin to see why a packet of (long shelf life) soup and a bread roll start to become attractive! After all, what is really being sold is a temporary tummy filler, to stop the growls, until you can get to the Burger King or whatever at your destination.