Why are there so few Lancasters still flying?
I can give you lots of reasons, but as with most things they all really boil down to one: MONEY.



Perhaps the first challenge with preserving any classic warbirds in airworthy "flying exhibit" condition is finding one, but let's assume you can locate a mostly-intact Avro Lancaster (in a field somewhere, or as a static display at a museum that hasn't done anything to it that would render it unflyable).

The next major hurdle is restoring it to airworthy condition economically: 
A side effect of their role in the military is that these aircraft are flown hard (they have to accomplish their mission and a little airframe stress or engine damage in the process is OK as long as you still get it back home in mostly-serviceable condition for repair), and of course they do tend to get shot at (9 out of 10 structural engineers surveyed agreed that bullet holes are a Bad Thing for airworthiness; The 10th wet himself and ran out screaming). They've also been sitting around for decades suffering the effects of time (corrosion).

Once you find the aircraft you have to determine what repairs it will require to render the airframe airworthy. You will probably be replacing skin panels, control cables, hydraulic and pneumatic lines, etc. at the very least. The longer it's sat, and the worse the environment, the higher the chance of more major repairs (structural corrosion requiring ribs or spars to be replaced/spliced), and as these aircraft are out of production there are some parts you can't just get off the shelf: Custom fabrication for skins, control surfaces, ribs, etc. adds to the price tag.

Once the airframe is addressed you can look to the engines, since an aircraft without a powerplant is still just a static display.
Much like with the airframe you may find engine parts are hard to come by (the two airworthy B-29s, Doc and FiFi, both required a lot of effort to get working engines).  

Now that you have a working engine you need to outfit the cockpit -- Many of the instruments the aircraft originally shipped with were likely pulled by the military or intervening owners as service spares for other flying aircraft, so what's left in the cockpit may be non-functional or so obsolete as to be useless. If you want an aircraft you can safely/legally fly to shows you'll probably need to invest tens-of-thousands of dollars in outfitting it with instruments and radios that are some functional combination of "authentic" and "modern".

Finally once it's all assembled, airworthy, and navigable you still have to pay to operate and maintain it like any other aircraft. When you're talking about a Lancaster with 4 fat engines on it you're racking up quite a fuel bill (and burning a good amount of oil with it).
The engines will of course need routine maintenance (oil changes, etc.) and eventually things will wear out or break, requiring repairs and leaving you with the "out-of-production parts" issues again.



Most of the "big" flying warbirds like the two airworthy B-29s and the two airworthy Lancasters are maintained by organizations that do it as a labor of love &amp; out of respect for the history of these aircraft, but they do not have unlimited funding and there aren't a large number of near-airworthy airframes to choose from for these projects (with the pool of available hulks ever-dwindling), so in that respect there's certainly a financial factor as to why there aren't more of these flying. 

Psychologically there's also probably something about having "at least two" airworthy examples flying that may make folks look at restoring a different type as well: Having two flying examples of aircraft from this period provides a decent level of insurance in so much as if one is lost in an accident or develops a problem that can't be fixed at least there's still the other one flying. (When FiFi was the only flying B-29 there were some folks who felt it shouldn't be flown, as an accident would cost the only airworthy example. Doc joining the flight line has muted some of those objections.)