Yes, they were. They had a lower TCO, their parts cost less and they required less maintenance than most of contemporary econoboxes. Auto parts stores loved them because they were cheap to buy, cheap to own and cheap to run. NAPA still has a huge fleet of the things.
Cheaper?
The currently cheapest Chevy pickup starts from 17k, to deliver pizzas a 12k Aveo would suffice. That's not cheaper, that's about 50% more expensive.
S10s haven't been sold in a long time. When they were, they could be had in fleet configurations for under $8K. This was when the cheapest car you could buy from Chevrolet was the $9K Cavalier, which shortly thereafter was used for 'buy one get one free' promotions. The Cavalier was notable for being cheaper than the competition, to give you an idea, because like the S10 all the tooling and such had been paid off long ago.
So, no, they were cheaper. The replacement Colorado/Canyon twins are an attempt to go upmarket with their truck to match what Toyota and everyone else has done and it's been a pretty big failure.
Less gas?
The pickup mentioned above does 18mpg city, the aveo does 27mpg city. That's not exactly "about equivalent" by any standard, that's 50% more mpg.
At the time, the Aveo wasn't offered, the then-just-discontinued Geo Metro got better MPG but broke more often (what good is a car that gets 30+mpg in the city but needs a new transmission on a yearly basis and a front suspension overhaul every month?), and in fleet config and service an S10 4 banger with the 'economy' rear end would break 21mpg. The Cavalier was about the same (same engine).
The scooter is a decent idea, some deliveries around here use them. Also solves the parking issue, just stop right in front of the house/shop. Those who use cars use VW Foxes, Hyundai i10s, etc. - more than enough car.
Nobody's saying you
need a truck to deliver your normal sized pizzas on a regular run. It's just that for a business, sometimes they do/did make more economic sense.
Put them flat into the trunk. They may slide around, yes. They won't topple over though. If they are put onto a seat any braking will make them slide forwards and topple, resulting in a messy pizza. Sliding around flat in the trunk won't mess up the pizza.
You clearly have never had a job delivering food. They DO flip up if put in the trunk. Trust me on this.
And unlike the cab, there's no belts you can attach the tie downs on the pizza box carrier bag to in the trunk.