Yes, it is a lot easier to get Harley parts. But it is a matter of company size more than anything else.
Suzuki sells about 15,000 cruisers/ touring bikes a year, and with the exception of the C50/ M50 semi-sisters they have almost no shared parts. Carrying inventory costs money, so you don’t want to stock anything you won’t sell fairly quickly. So simple ‘supply and demand’ dictates that as a dealer there is no point carrying a large parts inventory. Even if you had the space and money to stock one or two of everything for every bike, you’d probably never sell most of it. Suzuki sells a lot more sport bikes than cruisers, so as a dealer you’d tend to carry a lot more sport bike parts than cruiser parts in your limited inventory, and stick to just the most commonly needed cruiser parts.
On the other end of the spectrum is Harley Davidson, which sells about 350,000 cruisers/ touring bikes a year, With the exception of the metric V-rod and the Evolution Sportster models, they all share a big percentage of their parts. So simple ‘supply and demand’ dictates that as a dealer you NEED to keep several of everything on hand, because you will be selling a LOT of parts. It also dictates that each dealer be larger, and that there be a larger dealer network: Harley has over 1300 dealers and several thousand authorized service centers around the country. Plus there are probably 10,000 independent shops that can work on them for you, and hundreds of companies making replacement parts for all those bikes, so chances are no matter where you go, there you are.
So while Harley parts are a lot easier to come by than Suzuki parts, it is mostly just because Harley is bigger.
Buy a bike because you love the bike. If parts are easier to find just consider that a nice secondary benifit.
Rent an Electra Glide Classic for an afternoon. By the end of the day you'll either love it or hate it.