The Wikipedia webpage on Small-lift launch vehicle gives a huge list of dedicated launch vehicles with a maximum payload capacity of 2,000 kg (4,400 lb) for small satellites, which are either retired, operational or under development.
Almost all space agencies, to the best of my knowledge, started launching satellites using these kinds of rockets. For example, SpaceX started their journey with Falcon-1 (A Small-lift launch vehicle) and not with Falcon Heavy (A Heavy-lift launch vehicle). Of course, they learnt from each launch (both failures and success), and using these lessons they developed Medium-lift and Heavy-lift launch vehicles. There are some new countries like Spain, Argentina, etc., developing small-lift launch vehicles.
I don't understand why some countries with very reliable medium-lift and heavy-lift launch vehicles, again start developing small-lift launch vehicles. Examples include the Indian Space Research Organisation (ISRO) which is developing its Small Satellite Launch Vehicle (SSLV) even though it has operational medium-lift launch vehicles. Further, some countries have operational small-lift launch vehicles in addition to very reliable medium and heavy-lift launch vehicles. This list includes the United States, Russia and, China. There are some private companies like Rocket Lab and Firefly which are having either a fully operational or under development small-satellite launch vehicles.
Why hasn't the small-lift launch vehicles completely replaced by the medium and heavy-lift launch vehicles?
The main reason said in many sources is, using small-lift launch vehicles, small satellites (cube-sats, nano-sats, etc.) get a first-class ride rather than piggybacking on a medium or heavy-lift launch vehicles along with heavy satellites. I agree that sometimes these small satellites are secondary to the primary satellites when the success of primary payload is hindered due to a small malfunction in the launch vehicle. In one of the CRS missions, the secondary payloads were left in a lower than intended orbit due to a problem in the launch vehicle. These re-entered the Earth's atmosphere withing days after orbit-injection. Yes, in these cases it seems to be better to have small-lift launch vehicles. But, I am confused by the following question.
Why not utilise a medium-lift launch vehicle to launch only small satellites as primary payload?
Why do we need to develop a new dedicated launch vehicle for smallsats when existing launch vehicles can carry them into orbit? According to this answer for the question Why is SpaceX not creating its own launch vehicle for small satellites?,
They already have one. The Falcon 9. Earlier this month a single Falcon 9 put 64 smallsats on orbit. It was arranged by a rideshare company, Spaceflight, at prices that small launch vehicles would have a hard time competing with, starting at \$300,000.
Even if it were on the basis of cost, existing launch vehicles can tackle it easily based on the above example. If the entire system is fully reusable like the Starship and Super Heavy, then the costs reduce dramatically, and even the turn around time will become much shorter. Or in other words, the higher frequecy of the launch can also be handled easily, with little to no refurbishment.
If different CubeSats require different orbit requirements, why can't we just club the launches of CubeSats having the same requirements? Or in other words instead of say 10 small-lift launches to 400km polar orbit, why not have a single medium-lift launch to the same orbit?
Not only these super heavy-lift launch vehicles can launch small satellites but also very larger ones. Is this not a bonus? On the other hand, small-satellite launch vehicles can only launch small satellites and not big ones.
Why do we need dedicated launch vehicles for small satellites? Are there any other variables to play here? Do these rockets have any advantage over medium and heavy-lifters?
Another analogy to this is, Are we using small aircrafts like Cessna for air travel? No. We use bigger ones like Boeing 747, etc. The first-class ride on Cessna is much costlier than a first-class ride on an Airbus A380.
If it is not clear to you, kindly let me know in the comments. Thank you for reading.