Why is there such a small supply of dark red 1x4 bricks?
I don't know if there are any particular issues with manufacturing 1 x 4 bricks in Dark Red, but I suspect not. Instead, to understand why dark red 1 x 4s are relatively rare, it is interesting to consider why other colours might be relatively popular.

Popular Colours

Younger children like strong primary colours + black and white. At the same time, older children rarely object to primary colours. For this reason, there is a bias in LEGO's colour selection towards the primary colours. For this reason, Red, Yellow, Blue, White and Black have been part of the LEGO spectrum for a long time. This means that there is already a large number of bricks in these colours in circulation. Indeed (at the time of writing) there are well over 1,000 1x4s in each of these colours available on BrickLink. Green bricks have not been around quite as long, but you would expect them to be popular with children for a similar reason. Indeed, there are a good number of these available, too (over 700 lots today).

Space, Star Wars, castle etc. have required lots of Greys, so these are available in larger quantities. As you would expect, these are also available in larger quantities (over 700 lots each for the modern greys, around 300 - 400 lots for the older greys).

You would expect newer, bright colours to be popular with children. In addition, Lego aimed at girls has resulted in increased quantities of pinks, purples and pastel shades. This would account for why Orange (600+ lots), Lime (500+), Dark Purple (250+),  Bright Pink (250+), Medium Blue (250+), Dark Pink (100+) and Purple (50+) are relatively popular, but less common than the primaries and the greys.

Transparent 1x4s used to be used as windscreens etc. in older models, so there are quite a few of these in circulation (200+ lots on Bricklink).

Less Common Colours

Earthy colours - browns, tans and - have been required for a variety of models, typically representing desert or other natural phenomena. Tan, for example, has been a feature of Pharoah's Quest, various Star Wars sets, etc. This is why there are relatively large quantities of Tan (600+), Reddish Brown (600+), Dark Green (200+), Brown (100+) and Dark Tan (50+) lots available.

The quantity of Maersk Blue (100+ lots) is a special case, in that the licensed models that require this colour also require 1x4s in larger quantities.

Rare Colours

Then there are the rarer colours for 1 x 4s, where there are less than 50 lots of each colour available on BrickLink at the moment. These include Light Yellow, Medium Orange, Light Purple, Pink and our friend Dark Red (+ a few others). I suspect that there are so few of these simply because TLG have had not identified a market for models that contain 1 x 4s in these colours.

Ultra-Rare Colours

For completeness, it is interesting that there are a few colours that are ultra-rare (&lt;10 lots available) in 1 x 4 brick format: Magenta, Transparent Green and Chrome Gold, for example. Sometimes these ultra-rare bricks exist because they were used in a small number of sets that are themselves very rare, or because they were prototype bricks, because they were used in "special" models like the ones in theme-parks, or because the person listing them on BrickLink has misidentified their colour.

(The astute reader will notice that I've omitted Dark Blue, Sand Green and Sand Blue from this analysis, each of which has 200+ lots of 1 x 4s listed on BrickLink as of today. To be honest, I have not yet investigated their relative popularity compared to Dark Red. Suggestions would therefore be welcome.)

Conclusion

In conclusion, I suggest that the relative scarcity of Dark Red 1 x 4 bricks can be explained in terms of TLG's marketing strategy, both current and historic. TLG has always sold bricks in sets that are designed to appeal to children, and its marketing people appear to think that children are disinterested in a product that contains large numbers of Dark Red 1 x 4 bricks.