In Italy, the electoral system can be changed with the majority of the Lower and Upper houses. The current electoral system called by Giovanni Sartori "Porcellum" gives the majority (and a bonus of elected, "premio di maggioranza") to a coalition of parties, not to a single party, so small parties can survive. Some parties of those are present only in some territories.
Post-war Italy hadn't a pure majority electoral system, the current one is proportional and the previous (Mattarellum) was a mix (75% majority, 25% proportional).
During last 10 years, only two parties become strong enough (more than 20%) to lead coalitions: PDL (center-right) and PD (center-left).
So, why there are too many parties?
Electoral system can be changed by majority: if a party knows it'll lost next elections, it can change the law to limit the opponent.
No real majority electoral system was adopted in Italy after WWII.
Highly polarized electorate.
No serious election threshold for parties was adopted to correct fragmentation by proportional system.
In the Italian edition of Comparative Constitutional Engineering (Ingegneria costituzionale comparata) Sartori describes these problems very well, and many times he cites Germany (proportional system with per-party election threesold) as an example to follow.