It is because of the male chauvinistic people in the society who have snatched all the religious right from the women including that of the vedic studies and vedic rituals etc.
In the recent centuries we saw that widow's suffered a lot of torture after the death of husband. At some places they were tonsured, they were kept isolated and forced not to wear make-up or jewelry etc. which is unethical.
scriptures allow widows to remarry if they wish. (Rg Veda. X.18.8) (Ath Veda XVIII.3.2) Some Sutras allow a widow to marry her brother-in-law which is also stated in the Mahabharata though marrying brother-in law is not compulsory. Atharva Veda prescribes a ritual (ajA paNchodana) for lifelong union with the new husband. (AV 9.3.27). In MBH there are reference of widow remarriage. Arjuna had a son with the widowed daughter of Naga king airAvata. UgrAyudha wanted to marry Satyavati, the widow of shantanu.
In terms of rituals, both widow and widowers have almost same rules. There is no partiality between a widow and a widower (vidhura). A widow can perform the pArvana shraddha at sacred places during occasions such as mahalaya. She is not debarred from performing Vriddhi shraddha (ref. Shraddha manjari).
There are no injunctions in the vedas or scriptures like MBH or Puranas where we see widows getting tonsured hence such practices are later day additions. the Do's and Dont's are almost same for both wife and husband.