Why older coins are no longer minable for the average public [ Pre-mined coins] ?

Private pre-mine - When a coin is mined before it is made publicly available.
Public pre-mine - When a coin is launched with no advanced notice and a low difficulty.

There is a huge advantage of developers and their close circles over the public.
Moreover, any new coin developers should also choose a reasonable starting difficulty and reasonable advance notice of release.

Since most of the mining profits are made a the start of the release when the difficulty is low enough.
If the developers start difficulty too low people should consider it deliberate and 'effectively' a pre-mine if no notice of release has been given in advance.

These developers and their close circles mine most of the coins and sell them off when all coin have finished becoming mined.

These are also know as ScamCoins.

The reasons include:-

1. Unjustified premine
2. Disproportionate amount of coins generated at launch
3. Initially released in a remote corner of the web
4. Broken client at release
5. Developer incompetence
6. No Community

List of scamcoins – do not use:
PXC Phenixcoin: premined 1,000,000 coins
BTB Bitbar – premined by small group of coin creators
MNC Mincoin – started with no binaries and 25000% reward blocks
GLD Goldcoin – 2000% superblocks at start (3.2% instamine)
AMC Americancoin- premined with 6.5k-14k blocks giving zero reward
LKY LuckyCoin – superblocks at start, random superblocks
MEM MEMEcoin – premined, superblocks
ARG Argentum – premined
StableCoin – 1,200,000 premined
NUG Nuggets – premined
CDC Cloudcoin- 1,000,000 premined, random blocks
CENT Pennies – hidden launch
SYC Skycoin – premined
CL Copperlark – massively premined
NAN NanoTokens – premined
MST MasterCoin – premined
USC UScoin – premined
GIL – premined
RCH RichCoin – premined
XNC XenCoin – premined
REC RealCoin – premined
RED Recoin – 1,000,000 premined
ALF Alphacoin – premined
One Last Coin – premined
ORB Orbitcoin – 66% premined
NUC Nucoin – premined

Source
i. https://bitcointalk.org/index.php?topic=178697.0
ii. xorxor’s list of all cryptocurrencies at bitcointalk forum.
iii. http://cryptolife.net/the-anatomy-of-a-scamcoin-7-things-to-know-before-investing-in-an-altcoin/