First, I don't think that the attacks described in this article are meant to be in order. The only claim I can find is that these are the 10 most common attacks, but not that the first listed is more common than the others [EDIT: you've changed your question and no longer claim that the article says it is the most common attack].
But let me address your question from two sites: 1. is the article a reliable source in the first place and 2. why are DDoS still such common attacks.
As for the first: I doubt that all the attacks mentioned in the article are really the most common attacks. I rather suspect that this is what the author has just come up with when he was asked to quickly write some blog article. I doubt that there was actual research done about which attack is more common then another one - at least no such research is cited. Especially I highly doubt the claim that birthday attacks against hashes (number 9 in the list) are actually that common, compared to things like malware attack (number 10 in the list).
In other words: this is primarily a marketing article (written by a sales manager) and not a well researched technical article. One should be very careful to draw any conclusions from this article or even treat the claims as facts.
But it is still true that DDoS is one of the major attacks vectors. These attacks are easy to do: either the attacker has built a botnet already (see for example about the Mirai botnet) or the attacker can rent one or the attacker can simply use DDoS as a Service. This means they are not only easy to do but often also cheap, especially against small, less protected targets. And they are lots of these: players within online games, small companies ...
Stil, you claim that "There are many technologies and hardware out there to mitigate this kind of attack.". This is about specific as claiming that there are many therapies against cancer. Yes, there are but they are costly, cannot be used against all kinds of cancer and have usually unwanted side effects. And the same is true for Anti-DDoS techniques - they are not perfect and they are not free (of cost and of side effects). Thus they are not universally employed or even if employed provided only limited protection.
Apart from that, your statement "... many technologies and hardware out there to mitigate this kind of attack." is also true for most other major attack vectors. And similar is my statement about these mitigations not being perfect and free, and that's why they are not adopted everywhere.