Peers had passed amendments to the EU Withdrawal Bill that would have given parliament another vote at the end of the negotiation process and unilaterally guaranteed the rights of EU citizens remaining in the UK after we leave. Having already triggered Article 50, the only "meaningful" vote in parliament would been whether to accept the terms negotiated with the EU or whether to leave with no deal. The unilateral guarantee of EU citizens' rights would undermine future negotiations on the rights of UK citizens which are currently being blocked by several EU member states.
The decision not to force a game of parliamentary ping pong with the Brexit bill means that Theresa May can trigger Article 50 in time to avoid the need to pass legislation to prevent EU elections in 2019. We are told that Conservative MEPs currently sitting in Strasbourg believe that Article 50 will be triggered today although official sources are suggesting it will be at the end of the month.
If Theresa May does wait until the end of the month it will fuel speculation about her constant dithering over something she has promised to do since becoming prime minister. She has dithered and delayed for over 8 months allowing Remoaners time to launch vexatious legal challenges which have caused even more delays and produced even more opportunities to water down or block progress. There is a widely held belief that despite promising to deliver Brexit, the Remain-supporting prime minister is in fact trying to give her side as many opportunities as possible to scupper Brexit she can get away with.