(1) Your question is asked by the Darkei David who writes that it requires further research to understand why this statement of Rav Yochanan appears here in the Gemara. The Darkei David does not suggest an answer to this problem. I saw that the Otzar Meforshei ha'Talmud and the Hebrew Artscrolls both cite the Darkei David but neither found any other commentaries who attempt to explain why Rav Yochanan's Halachah is mentioned here.

(2) However there is no Beis Hamedrash without a Chidush, so I am going to try and offer my own Pshat, b'Ezer Hash-m, to explain why Rav Yochanan comes in here. I got the idea from what I saw in the Notes of Rav Yosef Shalom Elyashiv, Shlita. He points out that one must say that the buyer said explicitly that he is buying the cow for ploughing, because otherwise the seller could claim that he sold it on the understanding that it was being bought in order to be slaughtered, and therefore it would not matter that it possessed these drawbacks because anyway it is going to be Shechted very soon.

(3) Since Rav Yochanan is discussing someone who bought a cow specifically for ploughing, we can now understand why his Halachah is mentioned here in the Gemara. This is because our Mishnah deals with a cow being rented specifically for ploughing. Therefore once the Gemara concludes discussing Halachos concerning the cow rented out for ploughing, it proceeds to Rav Yochanan's Din about the cow sold for ploughing. Only when the Gemara finishes discussing this, does it leave the subject of hiring and selling cows, and pass on to the subject of renting donkeys which is the next Mishnah on 80a.

(4) In short, our Mishnah is the place in Shas where renting and selling cows for the purpose of ploughing is discussed, which is why Rav Yochanan comes in here. [Even though the Gemara Bava Kama 46a and Bava Basra 92a discusses a cow that was sold and was found to be a gorer, and there is a dispute between Rav and Shmuel whether the seller can argue that he sold it for slaughtering, nevertheless our Mishnah is a better home for Rav Yochanan's Halachah because our Mishnah talks explicitly about a cow rented clearly only for ploughing].

Here is another reason why Rabbi Yochanan's din is mentioned here in the Gemara. It would appear that when they learnt these Halachos in the Yeshiva of the Amoraim, they learnt the Din of R. Yochanan together with the above halachos concerning the tools attached to the cow which broke. Therefore when Rav Ashi recorded these Dinim in the Gemara he also mentioned them together because this was the way they were learnt in the Yeshiva.

Kol Tuv

Dovid Bloom