One reason might be that most people simply do not use LinkedIn as much as Facebook. This means there is less ad inventory. If someoone has a title "Marketing Director" but rarely logs in, they are not reachable via ads so LinkedIn will not include them in the potential reach. I have contacts who get back to me months after I emailed them via LinkedIn, apologizing that "I rarely check LinkedIn, so I am only now seeing your message".
I don't know a lot of people who have had success with LinkedIn ads. Everyone I've asked seems to feel they are a waste of money. We tried them for a client in 2014 and the minimum was $2 per click with very low volume. Maybe if we had bid $4 per click as they suggested we would have got more clicks but it was not worth it. Only Neil Patel (maybe a few others) seem to think they are worth it. You may want to check out this piece he wrote. Maybe they will work for you:
I would suggest using Facebook for this or running a Google display ad campaign on sites that these people are likely to frequent.
You could also try Taboola and Outbrain but you would need to create a compelling content piece. Both are content amplification networks and will not accept straight ads.
I've heard that Taboola is effective for reaching business audiences (they cookied these people to death!) but I believe you need to spend a minimum of 5k per month (as of 2016, maybe things have changed since then).