The #1 reason for slow progress in mobile (cell) availability and data speed in Germany was not even mentioned in the article: Radio licenses for frequencies in UMTS and LTE were auctioned by the state, and telecom providers had to pay absurd sums to be able to even participate. On top of this financial burden of course they had to invest into their infrastructure. As companies have to make a profit to survive, this lead to high prices and slow expansion of infrastructure.
Reason #2 is probably Deutsche Telekom. This is the privatized prior state monopoly and they profit off of their dominant position in the market and some extra regulation in their favor. They still operate much like a government agency and this stifles progress and makes it hard for other companies to compete.
TL;DR it's an absolute shitshow