Autofill is tricky. You're right, we should probably do more to make support a compelling position. That might not fix the problem though. The entire population of players isn't as homogeneous as it might seem at first. At some MMRs and times of day, we have more than enough supports in the system but not enough junglers or top laners. Position distribution also varies by region.
Warning: we're about to get pretty technical. 
Strictly speaking, Autofill isn't the only solution to position starvation, but we believe it's the best one we have thought of so far given our goals. To understand why, we have to dig into the tradeoffs of matchmaking in general, which are matchmaking speed, player agency, and game quality.
Matchmaking Speed
How long does it take from the time you click Find Match to when we transport you to Summoner's Rift for an epic five-on-five clown fiesta. Sometimes we can improve this by optimizing our algorithm.
Player Agency
How much control we give you; position preferences fall into this bucket. Autofill also affects player agency - it means you don't get to play the thing you picked this game for some reason (which we'll get to shortly).
Game Quality
How competitive your games are. We want to match you with people of similar skill so both teams have a good shot at winning the game and it doesn't feel like a total stomp.
The easiest way to talk about the tradeoffs among these is to look at game quality. If we wanted to make matchmaking as fast as possible, we might be ok with lower game quality. If we matched you with basically any other player, we could probably make matchmaking pretty fast. For League, we typically choose to optimize for high quality games, which means longer matchmaking time because there are fewer "compatible" people to match you with.
So how is this related to Autofill? I mentioned above that Autofill is a lever we have that takes away some player agency. The position preference feature lets us give you more agency, but also makes the pool of compatible people smaller, so it comes with longer matchmaking time. Let's say we always wanted to give players their primary position. We could potentially do that, but we would likely need to trade game quality, matchmaking speed, or both.
When we know there's high supply for both your first and second preferences at the moment, rather than putting you in lower-quality matches or making you wait for a long time (in some cases very long), we make the choice to trade some of your agency and assign you to a higher-demand position.
This is probably WAY more than you ever wanted to know about why we have Autofill, but I hope it helps you understand how we look at the tradeoffs in matchmaking. We've talked about doing some long-form posts about our matchmaking system. If you guys are interested in knowing more, let us know!