Dear ivar52,
When expire of a parent, the great great grand parent get relived. (as we follow 3 generation system). As the marrige/ upanayanam ..etc are not to be carried out without Vriddi Nandhi, which means a start of a new era with pitru's blessing.
The vriddi nandhi are not performed till the passed soul gets a place in pitrulokam. (Till that, it is called pitru bootham) Which is considered to take one year. Untill then elder son supposed to conduct SodaKumbam everyday, 12 masikam, 4 unaabdikam & aabdikam after the end of first year. As the passed away soul is not attained pitru position hence it is not perfomed.
Also there is feeling that the pitru's are most angered & powerful blesser. Pitru kopam / shaapam will not provide a good result for the future generation.
Also the reason that, we may have to face lot of emotional embracement during the event as explained in the senior's reply.
Some arguments says that, the bachelor cannot perfom homam for pitru,hence he can get marry. But these are exemption in some situation.
In general, the marriges are not carriedout due to emotional issue & can be performed under some exempted situation, which need to be checked on timely basis.