Why can detecting middleweight black holes be harder than the other types? This source says that, until recently there were no middle weight (i.e. with 1000 solar-mass) black holes discovered. Astronomers had evidence of small and super-massive black holes, but not for the intermediate group. Now, at least 10 middleweight black holes are confirmed to exist.
Detecting black holes is difficult, especially the smaller ones. So, it should have been relatively easier to find middleweight ones than smaller ones, however, it seems not the case. 
Why?
 A: I think there could be many reasons. Stellar-sized black holes are common. They are likely the end-points of very massive stars ($>25 M_{\odot}$) and seem to be clustered at masses of $5-15M_{\odot}$. Somehwat larger examples might be formed by mergers. They are often easy to detect because they exist in binary systems; they accrete mass and emit X-rays, and their influence on the binary companion can be observed and the black hole mass inferred. An interesting aside here is that we knew nothing about the possibility of $30-60M_{\odot}$ black holes until they were detected via gravitational waves, and would otherwise have gone un-noticed. Mergers of black holes at these masses falls right in the "sweet spot" of the sensitivity of current GW detectors - i.e. the peak amplitude of GWs occurs at frequencies of 30-300 Hz just before merger.
Supermassive black holes $(>10^6M_{\odot})$ are found in the centres of galaxies. They are detected via the accretion of gas, which causes the "active galactic nucleus"/quasar phenomena and the dynamics of the gas can be used to estimate the black hole mass. More massive black holes lead to more extreme objects with higher AGN/quasar luminosites and faster gas motions that are easier to detect. Detection by gravitational waves is not yet possible for these objects because the frequency of the waves at merger is at very low frequencies that are not possible to probe using Earth-based detectors.
There are therefore two clear selection biases - nearby, stellar-sized black holes are readily detectable because they are common and their presence can be determined when they are in binary systems (which are also common). Distant black holes at the centres of galaxies are more readily detectable if they are massive.
Intermediate-mass black holes fall into the gap. They may have been formed early in the universe (the mechanisms are still not clear) and probably exist in the centres of relatively small, inconspicuous galaxies. Most may have gone on to grow or merge into the more supermassive black holes, so they may not be common in nearby galaxies at the present epoch of the universe. Secondly, because they are not very massive then their presence does not generally lead to highly energetic phenomenon. The luminosity of any AGN-type activity could be very small (as it is in our own galaxy, even though it hosts a 4 million solar-mass black hole), so they may not be readily identified. Even if candidates are identified it is hard to infer masses because any radiation due to gas motions will be correspondingly weaker. Finally, their masses mean (think about Kepler's third law) that their orbital frequencies at merger (if they are in binaries) are $\ll 1$ Hz
and they are currently undetectable as gravitational wave sources.
A: In a nutshell, intermediate-mass black holes (IMBHs for short) cannot be formed by the collapse of a star, which is how stellar mass black holes are formed, and can't be formed from the extreme conditions that form supermassive black holes. The three proposed methods of formation of IMBHs are:


*

*The merging of two or more stellar mass black holes.

*Collision of runaway stars, which then collapse.

*Primordial black holes from the big bang.


The last option of formation is a particularly interesting possibility and is an open area of research (I think Alan Guth's group is doing a great deal of research around it and how it relates to inflation). These are all pretty seldom-occuring events, amd so IMBHs aren't predicted to exist in as high numbers as other kinds.
I'm no expert in this field, so corrections/comments are appreciated.
I hope this helps!
A: The simple answer is, as you suggest in your comment, that low-mass black holes are probably much, much more common, which by itself makes it a lot easier to find one that happens to be accreting (and is thus visible to us). A very crude estimate would be that there are at last ten million (possibly as many as a billion) low-mass black holes in our galaxy.[*]
An additional factor might be that some of the low-mass black holes will form as part of binary star systems, and in some cases the secondary star will be close enough for some of its matter to accrete onto the black hole when it (the secondary star) evolves and expands in radius, thus creating the conditions for us to be able to detect the black hole.
[*] There are several hundred billion stars in our galaxy. If only 1 in a thousand stars forms with enough mass to produce a black hole at the end of its life, then there would be several hundred million black holes. Using a more sophisticated analysis, this paper predicts several tens of millions of black holes with masses $> 10$ solar masses in a Milky-Way-sized galaxy.
