There are two main reasons why DIPAs are generally more bitter than IPAs:

Increased Hop Quantity:  DIPAs are brewed with a significantly higher amount of hops compared to IPAs. These hops contribute bitterness to the beer. The additional hops in a DIPA lead to a more pronounced and intense bitterness.

Higher Gravity:  DIPAs have a higher alcohol content (ABV) than IPAs. This higher gravity can accentuate the perception of bitterness on the tongue. Even though the bitterness level per volume of beer might be similar between a DIPA and an IPA, the higher alcohol in the DIPA can make it taste more bitter overall.

Here's a breakdown of the factors contributing to DIPA bitterness:

Bittering Hops: Early additions of hops during the boil are primarily for bittering. DIPAs use more hops for bittering, leading to a higher bitterness level.

Aroma Hops: These hops are added later in the brewing process and contribute to the hop aroma and flavor. While not directly adding bitterness, the intense hop character of DIPAs can further enhance the perceived bitterness.

Higher Gravity: The increased alcohol content in DIPAs can accentuate the bitterness perception on the taste buds.

However, it's important to note that not all DIPAs are overwhelmingly bitter. Some breweries strive for a balance between hop bitterness, malt sweetness, and hop aroma in their DIPAs. This can be achieved by using different hop varieties with varying bitterness profiles, adjusting the timing of hop additions, and carefully selecting malts to provide a counterpoint to the bitterness.