Fund Houses sometimes restrict the flow of funds in particular schemes by allowing only SIPs or lumpsum investments. This can be due to various reasons:
The AUM of the fund has gone too high and the manager wants to restrict more investments in that scheme.
Due to a bull run in the market, the fund manager may feel that the valuations are too high and does not want to make further investments in that scheme at the current prices.
To protect the interest of existing scheme holders in case of any underperformance due to the deployment of new investors’ funds.