Possibly physical safety issues, theyre really light, I wouldn't want to be in a crash in one. Mind you I may end up driving one myself regardless, just you asked why not. There are substantially safer cars without much mileage penalty/that get pretty close. If old cars aren't a problem and you arent as concerned there's a number of vehicles not far off
Super Cheap High MPG Cars: 1978-1981 | MPGomatic
Possibly maintenance or reliability concerns... I don't know about the raw statistics, but the impression I got from metro driving friends was they all started to fall apart pretty fast after 80k miles. The lower A-arms rust to crap in any non-southern climate, even potentially there if parked outside If not repaired you can find yourself heading into a wall at 70mph as it ignores your steering wheel - be sure to check these. The hondas definately have a better reputation for reliability - you often spend more for that reason. Theyre like appliances - they just run and run.
Fuel efficiency savings can be eaten up by other costs - there's more to life than ultimate efficiency.
Work out your own acceptable compromises and priorities. For myself my "best solution" would have been to get a CNG car due to the cheapness of it's fuel, despite that it has lower ultimate MPG... except that the home fueling station costs $5000 by itself and there's no commercial fill stations at a remotely decent price. If that ever changes, that alone, some place to fill it up, i'll be driving a CNG car very quickly. Probably a Cavalier. My backup plan is an SVO fueled IDI VW Jetta - which will be determined by the outcome of my own growing of oilseeds. The "free" fuel matters more than the ultimate MPG. If you have to run normal gasoline though then yes the metros seem to be about as good as it gets. Note that some people are charging much more - thousands more, more than you might save on gas even - to get decent quality ones. Finance and purchase costs are part of it too.
My own favorites are the Saturn SL's with 29city/40 highway. That's not the highest mileage but they are cheap to buy, cheap to repair if anything goes wrong, more reliable than practically any other domestic I know of, some better safety figures than a number of other compacts at that time, and the plastic sides mean less rust and parking lot dings. But only get a stick shift. I even think theyre more driveable as far as feeling in tune with the car than other cars i've tried of the late 90's - nice big gauges, handle well for what it is (i've driven plenty that didn't including hondas and toyotas of that era), etc. Not as good of mileage as the best honda but your not spending as much as a honda either.
What is your current mileage? $20/day sounds like more than 5 gallons a day but outbacks i'm looking at indicate 29mpg highway... it's possible you will not get the stated highway mileage over hilly territory. I'm also told that old Xfi's don't tend to get the "55mpg" for whatever reason - that i should expect more like 45mpg if i get one used. I haven't heard of used models of many other vehicles dropping as much just from age necessarily.
The jump from say 30 to 40mpg is worth 33% less cost than the next jump from 40 to 50mpg 25% less cost, and 50 to 60mpg 20% less fuel cost. If your really hardcore about best mileage there's really no alternative but aeromodding - for me I didn't want the car to look funny but i'm finally accepting if I want the cheap fuel I need to get over my hangups because it's more "am I willing to spend X more per month to NOT have the funny looking tail on it?" and put that way not really. See what some are doing in the quest for FE - some are taking safety compromises that I will not go for for instance.