Because that wouldn't be a valid comparison...that would be taking it too far in the other direction.
"US Based Extremists" is an extremely broad category, while "Islamic Extremism" is narrowing down to a specific ideology and thought process.
...and, there can be significant overlap between those two labels. For example, the San Bernardino incident would qualify as both since the aggressor fit both categories.
Anti-abortion terrorism
Anti-government terrorism
Anti-Semitic terrorism
Anti-Islamic terrorism
Anti-Gay terrorism
Racial Terrorism
Eco-Terrorism
Christian Terrorism
Then you have hate groups like the KKK who are a hate-based organization, but in terms of actual violent acts committed, have been pretty much neutered.
Then you have gangs who, in all reality, should be labeled as terrorist groups (based on kill count alone), but aren't...
etc...
...those are all brands of US-based extremism, and for each of those from 1990 until present day, you'd be hard pressed to find one that, by itself, has the same body count and number of high-profile attacks.
Even when you do the math on it, Islamic extremism (in terms of body count) is still way over-represented in terms of the numbers.
If you think of it this way. The Muslim population in the US is just over 3 million.
Based on polling data pertaining to certain social attitudes (on women, free speech, separation of church & stay, gays, secularism, etc...), only 9% hold views that would be considered concerning or "extreme". That means 270,000 may have views that could lead to concerning behavior (and in actuality, that number is probably much smaller because even if a person says they'd want to do something, very few would actually have the guts to do it).
That means that a group 270,000 (likely much smaller) is responsible for 20% of all terror attacks.
And, in my own personal opinion, that 20% number is actually low-balling due to a watering down of the word "attack".
If it's the same set of FBI stats I looked at about a year back. They lump property damage, vandalism, and minor assault in the same category as mass shootings and other events where there is actual loss of life or injury.
We can honestly look and say "yeah, spray-painting a Swastika on a Jewish temple is an act of hate, and it's an act intimidation"
...however, in terms of what's going to make the public scared/concerned/paranoid, spray-painting a hate symbol on the side of a building or 1-2 people getting beat up doesn't fall in the same category as the San Bernardino or the Orlando shootings.
I understand that people want to be sensitive to other groups, and they don't want to appear as bigots or politically incorrect on the topic of Islam...but as I've said before, to pretend that "other groups are just as bad", or watering down the term "terrorist attack" in order to pad the statistics for other forms of extremism so they can come out with the 20% statistic is intellectually dishonest and, as Sam Harris said, "being deliberately ignorant about the problem"
If you want to know why people are more scared of one than that other, you just need to honestly consider the following numbers (and for the sake of fairness, we'll ignore 9/11 since I understand that grossly skews the numbers):
Fort Hood Shooting: 13 dead, 33 injured
Boston Marathon Bombing: 3 dead, 183 injuries
San Bernardino Attack: 14 dead, 22 injured
Orlando Nightclub Shooting: 49 dead, 53 injured
Now let's compare that to some of the other forms of terrorism discussed above:
https://en.wikipedia.org/wiki/Terrorism_in_the_United_States
KKK - hasn't killed anyone since 1981
Left Wing Extremist groups - haven't killed anyone since 1983
Eco Extremists (like ALF) - property damage
White Nationalist Terrorism - Charleston Church Shooting, Wisconsin Temple shooting...body count of the two events combined: 16 dead, 5 injured -- prior to those two events, that last incident involving physical harm was years ago.
Anti-abortion attacks...most occurences, however, fatalities are rare and 90% of it is property damage.
If you notice, there's one example listed where it highlights what I was talking about before about the watering down of the term "attack".
1999: Martin Uphoff set fire to a Planned Parenthood clinic in Sioux Falls, South Dakota, causing US$100 worth of damage.
So, apparently causing $100 worth of damage is the same as 50+ casualties.