During the time of Buddha, it is said many people at one gathering attained enlightenment by listening to only one one of his sermons. How can a mind develop such a achieving within matter of hours. There are millions of devoted Buddhist in the world now but I have not heard a single person achieving higher mental status. Are there any other explanation this other than karma?
There are several traditional takes on this.
According to some commentary the Dhamma declines gradually so that there is a steady decline and progressive lack of higher attainments in the world until there are no attainments left. Based on this kind of reasoning some hold that there is a cap on what will be attained in our time, some hold that Sotapannaship is possible whereas other hold that as being impossible because they base it on a different calculation and values ie 500 years for disappearance of this or that attainment instead of a 1000.
Another popular interpretation is that Dhamma can be revived so that even if there is nobody with attainments during a decline, it is still possible to get all or some of the attainments.
These are the basis for popular theravadin positions.
To share one's attainments is generally frowned upon as we do not have the buddha here to verify attainments. However, in the vipassana tradition there are some lay people who have demonstrated extreme discipline for example: Dipa Ma https://dipama.com/ May find peace and happiness, With Metta :)
How can a mind develop such a achieving within matter of hours.
Sorta like math prodigies being able to master calculus, differential equations, or number theory when still in middle school. There's no difference, an inevitable result of many lives of cultivation and dedication to the discipline.
There are millions of devoted Buddhist in the world now but I have not heard a single person achieving higher mental status. Are there any other explanation this other than karma?
Probably because the real deal who's truly achieved those higher attainments wouldn't go around and proclaim it to others. Anyway, it's not necessarily true that modern men's intelligence or capacity are duller or dumber than their ancient counterparts, it's definitely true that the environment our ancestors once lived was vastly more conducive to the cultivation of the Dhamma. Back then, there were no TVs, radios, internet chatrooms, facebook, snapchat, twitter, zoom, tik tok, and a gazillion of other modern day distractions.
Samadhi(Concentration) should be developed, Sila(Morality) should be practiced and with the help of these 2, Paññā(Discernment) will be developed. Only by having discernment can one understand the Buddhist teachings. At the time of Buddha, as SarathW said, people who attained stages of awakening already perfected Sila and Samadhi therefore it was easy for them to understand the Buddha's teachings. In the present if you want to achieve 'awakening' you should perfect Sila(Minimum 5 precepts) and Samadhi (Samata Meditation) and move onto Insight (Vipassana) Meditation, with time you will awaken.
The reason today's laypeople and even Bhikkus do not achieve awakening is because they do not practice Buddha's teachings. The teachings have turned to lip recital but not practice.