Rojo - I presume there are a number of factors at play with Falmouth. This is just looking at its history. But I agree, if you look at the top 50 finishers from the 90s and compare it to today, the two are nothing alike.
First, there are a number of other road races both in the US and abroad competing for these same athletes. Races like Falmouth used to be part of the summer road-racing circuit of maybe 4-5 races that paid decent money. Now there are a larger number of races in the US, and newly minted races in Europe, where the runners do not have to travel as far and can earn a similar prize payout. I also think the reluctance by Kipchoge, for instance, to only focus on the marathon and not to run any other races, ever, has hurt the marketplace as other road racers mimic his training/racing plan.
Second, the marketability of the race does not change if you have a blazing fast time compared to a decent time with a handful of competitors filling out your top 10. This is something the race director has mentioned to the various races he has helped to organize over the years. You do not need to bring in dozens and dozens of paid pros to sell your race to sponsors.
Third, drug testing is a factor now too. Falmouth drug tests, so do other major races. I am not going to assume that all African runners were cheating, but for a long enough period of time it was easy for a runner (from anywhere, not just Africa) to dope. Not anymore, and that is a deterrent to some.
Fouth, travel costs/budgets. The races do not want to pay the cost to fly in a runner from Africa if they can avoid it. Previously, races only had to fly/bus in a runner from the last race they were in (usually Beach 2 Beacon). The travel costs have grown immensely thanks to COVID, oil costs, and the origination location for travel. Now that a lot of runners aren't all running the American circuit (because they may be in Europe, for instance), the races are forced to spend a lot more money to transport in the runners, which they do not want to do. Absent a prior agreement, Falmouth used to reimburse a portion of travel if a runner finished in the top 5 or 10. But that requires the runner pay his/her own way, and perform well. Otherwise they are out a lot of money. Athletes don't want to take that risk.
Fifth, prize money. If you compare the prize money for Falmouth now, compared to say the late 90s or early 00s, the prize money I think is LESS. I think for a period of time the first place prize peaked at $12,000 or $15,000. Today it is back to $10,000. And $10,000 does not go as far as it used to 30 years ago when the East African running boom began (and the prize money back then was $10,000, too). The prize money also does not go as deep for the number of top finishers, either. This year, the prize money was not even published, so who knows. The race board members declared that they are most interested in fundraising the highest amount of money possible. They charged about $100 to run this year, so they have probably maxed out entry fee prices for a while. Sponsorships look healthy but there is only so much money to be gained that way. They were forced to limit the number of bibs they could sell from 12,800 to 10,000, which cuts into the budget. So the next way to maximize profit is to cut costs, such as travel budgets and prize money.
Presently, as far as elite athletes go, the races will probably accept runners from anywhere. But appearance fee money is probably slim. Travel reimbursement is also slim, too. So if you are a runner training in Africa or racing in Europe, it is asking a lot to pay your way to travel to Falmouth, run a course that is tough on the legs through tough weather conditions, for a maximum payout of $10,000.