Many people from other Asian countries get shocked when they see the striking departure of Japanese Buddhism from the monastic and ascetic emphasis of most other forms of Buddhism. In Japan, you might even see a Buddhist cleric coming out from a strip bar joking that he just got rid of his kleshas. They have sex and get married, eat meat and drink alcohol......so, they don't really seem to have any strict lifestyle or following any Buddhist commandments. Today, 90% of the Buddhist clergy are married and more than 80% of temples are inhereted from families.
The Meiji Restoration is often reffered to for this trend of secularisation. Until 1872, marriage was illegal for Buddhist clerics by state law. The situation changed this year after the government declared that "from now on Buddhist clerics shall be free to eat meat, marry, grow hair and so on." They said that they would not interfere on Buddhist commandments, which the Buddhist comunity should decide themselves. There was a huge debate within the Buddhist community, but after a while, the majority of clerics took it positively and started to openly act as they do today.
However, this is probably not the major reason because the issue had been seriously raised before the Meiji Restoration, and it has been argued that violations of the commandments were one of the reasons for the rising public distrust in the early modern period.
The Edo era (17th-19th century) seemed to have had some effect on this issue. The government used temples as a substitute for local authorities to manage the public (e.g. family register was managed by temples). This led to an increase in financial stability for the temples, and a change in relationship with the public: it changed from a religious relationship to a more administrative one. As a consequence, the temple became manegable with a small number of people (making it easier to form a family), and religious attitudes seem to have changed (they were now part of the bureaucracy, rather than a religious institute).
Others refer to its close relation with political power (it is probably only during the medieval period of Japan when they were close to the general public). Rather than coming up from the public, Japanese Buddhism was imported and supported by the political powers. Therefore, clerics often tried to enclose people from politically powerful families and keep their family lines to posess political power. At some time, the major temples and high positions were full with these people, which helped making it morally acceptable to have a family as a monk.
Above all, the main discussions about this issue seems to lead to the fact that Buddhism was imported as a form of science or philosophy, and commandments were regarded as less important from the very beginning (6th-7th century). Often factors related to commandments were not included as a qualification of recieving formal license. It was not just the clerics, but also the followers tend to expect exciting lectures or some deep insights from the clerics rather than religious strictness.
Reference:
「現代日本仏教の特徴‐妻帯の歴史的背景を考える‐」蓑輪顕量（２００６）