For anyone interested, the problem mostly boils down to the sequence lenght we can read.
Modern sequencing techniques allow us o read 200 to 500 bases at a time. So after hat we need do find a way o arrange these short sequences into a single sequence. And this can be prety hard, especially when you are doing 'de novo' assembly[0].
Besides that, there is the fact that some regions of DNA are repeated[1].
[0] - https://en.wikipedia.org/wiki/Third-generation_sequencing#De...