They were developed by two different groups of companies.
If you can develop a new technology people have to pay you to use that technology. Two different groups of companies tried to produce the "standard" for DVDs, but in the end both got used so now we have both formats.
They are very similar and most DVD devices nowadays support both.
It was a bit like Blu-Ray and HD-DVD. This is the next generation DVD format, and two companies tried to develop the next "standard".
In the end the HD-DVD people gave up and Blu Ray has won.