What aircraft or airline differences would make the Sukhoi Superjet unsuitable for US regional airline operations?
This is some speculation, but one potential reason is that U.S. carriers already have fleets of Embraer and Bombardier jets in this basic size (the Su-100 competes directly with the Embraer 190/195 and Bombardier's larger CRJs and upcoming C-series). The companies making these jets are located in pretty firmly U.S.-allied countries that are unlikely to see any economic restrictions that might hamper the ability of U.S. regional carriers to procure new airframes or parts. 

The same cannot be said for Russia, which from 1946 until 1991 was pretty much embargoed by NATO signatories, and the relationship between Russia and the West since then has been testy. While the U.S. currently does not have a trade embargo with Russian civilian jet production operations (current sanctions attempt to specifically target military R&amp;D and direct investment), that can change in a heartbeat if Russia continues to throw its weight around in neighboring countries.

Second, the seating capacity of this jet is a bit of an odd duck, as it blurs the line between the needs of regional vs. national U.S. carriers. The basic problem is that regionals, by their very nature, operate out of smaller cities with fewer potential passengers, at distances from major hubs that makes driving to that larger airport and flying from there more palatable for passengers. These considerations cause passenger count on regionals to drop off dramatically compared to longer "national" routes. The largest thing American Eagle currently flies is a Bombardier CRJ-900 (75 passengers in 2-class config), with the most commonly operated jet in AE's fleet being the Embraer ERJ145 (50 seats all-economy). By comparison the smallest plane operated under the full American badge is the A319 (124 seats in 2-class config). 

There's an obvious gap between the two subsidiaries' aircraft capacities, but not much interest (by American at least) in filling it with a 100-seat jet like the SSJ100 (or E195 or C100). The primary reason is the collective bargaining agreements that pilots have with the airlines; they include, among other things, a requirement segregating the aircraft used by regional versus national subsidiaries. If you want to fly a plane with more than 80-100 seats (depending on exact terms of the agreement) on the route, you're paying a national airline flight crew the extra bucks to fly it; it's a violation of the CBA to put a 737 or A319 in the hands of a regional airline pilot because that takes the job away from the more senior pilots. 

A secondary reason is that U.S. domestic airlines like American tend to favor more, smaller flights over fewer larger ones. This allows the airline to "rightsize" the route schedule (and its fleet) to passenger demand more closely and with fewer disruptions. If on a given day of the week, 200 passengers need to get between a regional airport and a larger hub, the airline could do that with four 50-place jets, or 3 75-place jets, or two 100-place jets. Theoretically, if demand were constant, it could save money the fewer flights it made, as the cost of the 50-seat Embraer's two-man flight crew would be roughly the same as the Sukhoi's, but with the Sukhoi you spread that cost over twice the passengers so your gross margin is higher. 

However, an empty seat is a big fat goose-egg on the revenue for that seat for the flight, so if passenger demand varies (and it does, from day to day and seasonally), running larger jets either forces you to turn away some customers that you can't fit into the reduced schedule, or add flights that run mostly empty. Neither of these are palatable options for the airline; one pushes potential customers into the arms of their competitors, while the other has you running at least one flight that day costing tens of thousands of dollars for a handful of people with tickets whose regional component of the price might not even cover the pilot's salary for the duration of the flight (regionals are often "connecting" flights; DFW to PHX costs about \$320, ACT to DFW about \$300, but ACT to PHX via DFW only costs about \$360, so depending on how you part it out, the ACT-DFW leg of ACT-PHX might only really make you \$40 a seat, which might not break even if the plane's full, much less if it's half empty).

So, assuming any plane an airline would buy or lease is profitable when it's fully loaded with paying customers, American is more likely to successfully operate a route, and its entire network, using more smaller planes and scheduling a different number of interchangeable airframes and flight crews each day to maximize utilization, than it would be trying to also minimize the number of flights by using a variety of airframe sizes, which involves ensuring those airframes and the flight crews to run them are all in the right places at the right times (and deadheading - flying an empty plane from where it is right now to where it will be needed later - is bad no matter what the plane size, but typically more palatable with a smaller jet that has lower total operating costs).

Now, American isn't the only carrier with a regional subsidiary. US Airways Express and JetBlue, as well as several other New World regional carriers outside the U.S., regularly operate 100-seat Bombardier and Embraer airframes on regional connectors, and it would stand to reason that Sukhoi could get a piece of that action. It could also "stretch" the Superjet and try to compete in the national domestic market with the 125-seat A319 and 737-700 which are becoming more and more popular as U.S. domestic flights get "rightsized". However, the delicate nature of Russian-Western relations will probably continue to make Sukhoi a risky proposition for U.S. carriers that need a reliable source of airframes and parts on a 20-25 year timeline.