I think it is fairly evident to state that the reason why we don't see very many great shooters these days is because of the fact that technology has evolved well beyond what we had 30 years ago when R-Type came out. There is only so much graphical incentive that you can give someone to play a standard shooter these days and that is why not many of them end up in the forefront. However, 30 years ago or so shooters were a dime a dozen and the R-Type came along.
source
exceptionally crisp graphics for the time period
As was typical in shooters (and still is, for most part) the game consisted of side-scrolling levels with a large boss at the end of each. R-type didn't deviate from this and was for the most part celebrated for the introduction of those little pods that you would pick up and attach to or float around various parts of your main ship. This pod was immune to most type of damage and therefore would be extremely helpful in situations where a ton of bullets were flying at you.
At times it would be necessary (or at the minimum, extremely advantageous) to attach it to the back of your ship in order to be capable of firing backwards where enemies would emerge. There was no real method of knowing when this was going to happen, so it become a real plus if you were intimately familiar with the various level's layout.
R-Type (for the time period anyway) was extremely difficult and this kind of separated the serious gamers from the casuals. There was really no way to simply walk on and perform well since many of the various tunnels you could enter would actually be dead ends and meant certain death since backtracking wasn't a possibility (the screen progressed to the right whether or not you wanted it to do so.)
The first time I ever played this game was on a relatively short-lived system in the USA called the TurboGrafx-16. I was one of the few people I knew that even had one of these. R-Type sequels were later released on mulitple systems but failed to gather as much fanfare as the original 1987 masterpiece. The most recent addition was released on pretty much all systems in a reboot of sorts called R-Type Dimensions, which I never played. Hmmm, maybe i will... or is this one of those things that would better be left in my distant memories?
2-D shooters might be dead for the most part but there was a time when they were one of the most sought after types of game available in anyone's library.